The answer to this question lies in explaining why the occultation took place, what the awaiting of the Mahdi (aj) truly means, and what the benefits and outcomes of having a concealed imam are.
It has been said in our hadiths that one of the reasons for the occultation is so that he won't have to pledge allegiance to anyone, or so that he will be protected from being killed. Of course, these reasons can't be considered reasons for him necessarily being born a long time before his emergence, but there are other reasons which make such a matter for sure:
a) If the imam is to establish a new civilization unlike previous ones, it takes time for the people to realize that their own methods of life and civilizations aren’t up to the task of managing humanity and fulfilling its high goals and ideals, therefore people need to see such a matter for themselves in order to look forward to, be prepared and accept his universal government and see it as the solution to all their problems.
b) The occultation of the imam is a means of testing the servants of Allah (swt). Why Allah's (swt) servants need to be tested is the same reason for why there has to be an unseen imam that is a means of testing the people.
c) Next to the concept of occultation, there is the concept of anticipation and awaiting; the two are inextricably intertwined. Anticipation brings both personal spiritual development and social development, and although these two are reachable even without belief in a live and present (although unseen) imam, yet belief in a live and present imam that might emerge any minute, strengthens and speeds the two up.
In our hadiths, the imam during his occultation has been compared to the sun behind the clouds; it benefits all, yet isn’t seen. Benefits of this sun can be:
1- Just like how the commander of an army gives hope to his soldiers when he is alive and in the battlefield, a live and present imam gives hope to his followers as well.
2- Knowing that the imam is alive and monitoring our actions has a deep influence on individuals, causing them to work harder on building themselves spiritually.
3- All infallible imams and divine apostles, Imam Mahdi (aj) being the last of them, have been the guardians of Islam in their time, bearing all the secrets, reasons and proofs of Islam, both those that are apparent and manifest, and those that are concealed and hidden. It is on this basis that the protection and safeguard of religion and its concepts is their responsibility, and they are the ones who have to prevent Islam, which is the "seal of religions" as a result of the closing of the door of revelation to all after it, [hence, its superiority to other religions] from undergoing the slightest alteration and distortion, so it can be brought and applied to the everyday lives of normal people.
4- Some actually possess the capacity and are qualified to "climb the ladder" and ascend to the skies and make direct use of the sun instead of it being behind the clouds [meaning that they see or make direct contact with the imam, or might not see him, yet they are benefited in a special way that others aren’t].
5- We believe that the imam (aj) has authority over the hearts and actions of the people and in reality, guidance goes through him and it is his responsibility to help others reach their destinations, all of these are contingent upon the imam being live and present.
6- The aim of creation is the perfection of man, thus, if there is to be a day in which there is no perfect individual on the face of the planet, the world will cease to exist, therefore there is always a need for at least one perfect individual to be living on it, hence the need for a live imam.
7- In order for there to be a link between the realm of essential unity and pure multiplicity, there is a need for a third manifestation that serves as a channel between the two worlds, that channel being the perfect and complete individual, who is none other than an infallible imam.
The answer to this question lies in the explanation of what the reason for Imam Mahdi's (aj) occultation is, what it means to truly await his return, and what the benefits of a present imam [although unseen] are. But before getting started, this point must be made that the true main reason for Imam Mahdi's occultation, as mentioned in hadiths, is a secret that only Allah (swt) knows of exactly. Nevertheless, we can try to discover some of the other reasons behind it with the help of hadiths and our own thought. These reasons can be divided into two categories:
1- Those related to Imam Mahdi (aj)
2- Those related to other matters
As for those that have to do with Imam Mahdi (aj) himself:
a) It has been said in hadiths that the imam is in occultation so that there is no need for him to have pledged allegiance (bay'ah) to anybody.
b) It has also been said that one of the reasons is for him to be protected from being killed [just like how all our previous imams were].
c) He is to change and bring reform to the entire world and eradicate all civilizations based on oppression, materialism and treachery, establishing a totally new Godly civilization. Such a thing won't be possible, unless the people of the world experience different civilizations and see for themselves how those civilizations weren’t and aren’t up to the task of fulfilling the goals and ideals of humanity, hence the awaiting and eagerness of the people for change and the establishment of the rule of Imam Mahdi (aj).
The reasons pertaining to other matters:
a) Testing the people is one of the reasons mentioned in our hadiths. Imam Musa ibn Jafar (as) has said: "When the fifth son of the seventh imam goes into occultation, be very careful about your faith; don’t allow anyone to cause you to part it. O my dear son! There is no choice but for the Sahebul-Amr (Imam Mahdi (aj)) to have an occultation, [that will be prolonged for so long] to the extent that some of the believers will denounce their faith [as a result of doubt]. Allah (swt) will test His servants with occultation."
b) Awaiting his return factors into us being more pious and more careful about our spirituality, and helps us in our struggles against the self.
Awaiting his return is a concept that only makes sense when there is an occultation or absence. There is no awaiting unless there is an occultation. Awaiting a Godly government is made up of two affirmative and negative components; not being happy with the current situation the world is in, and looking forward to and loving a better one. If these two elements take root in one's heart and soul, two more things will stem from them: deserting all causes of tyranny and oppression, and physical and spiritual preparation for the one just government to come. These two are the agents needed for one's true awakening and movement. They are the factors that make the true awaiting of Imam Mahdi's (aj) coming a form of worship, and make those awaiting him like those standing in his tent or under his flag.
Awaiting a revolution that tyrants and oppressors aren’t welcome and have no place in, calls for the one awaiting such a matter to be pious and virtuous himself so that he will at least be welcome.
Now, considering this matter and combining it with what other hadiths tell us about how the imam is constantly monitoring his followers and takes a look at their actions every week, it can be concluded that believing in a live and present [although unseen] imam has a much greater impact on us rather than belief in an imam that is to come later and establish a just universal government.
c) Not only don’t true “awaiters” of Imam Mahdi (aj) lose their hope and become restless by the spread of corruption around the world, but [their faith strengthens and] they see this as a sign of the nearness of what they are looking forward to, thus never giving in to corruption or just going with the flow.
Although not losing hope and not giving in to corruption are also possible with an imam to be born in the future, but clearly, with a live and present imam, these two will both cover more area and will be stronger.
The benefits of Imam Mahdi's (aj) presence during the occultation
a) Giving hope: Belief in a live and present imam that might emerge at any time, is like the soldier who knows his commander is still alive and in the middle of the battlefield; it gives him hope and courage to continue fighting.
b) Spirituality: As was mentioned, considering the fact that Imam Mahdi (aj) monitors our actions by the week, this can have a special impact on individuals and them watching out for their acts, as some Quranic verses such as: "And say, "Go ahead and act [do what you like]: Allah will see your conduct, and His Apostle and the faithful [as well],…", and numerous hadiths that tell us about the prophet (pbuh) and imams seeing the acts of the good and sinners everyday, every Monday, or every Thursday, corroborate this matter.
Awaiting believers know that their actions are seen by the imam, and in order not to upset him or deprive themselves of his special attention and help, they are careful of what they do and try to do things that will please him more.
c) Guarding Allah's (swt) religion: In one of his great sayings, Imam Ali (as) explains the necessity of having a leader in every time and era: "But this earth will never be without those persons who will prove the universality of truth as disclosed by Allah, they may be well-known persons, openly and fearlessly declaring the things revealed to them or they may, under fear of harm, injury or deaths hide themselves from the public gaze and may carry on their mission privately so that the reasons proving the reality of truth as preached by religion and as demonstrated by His Prophet may not totally disappear"
As time goes on and people begin to add their personal opinions to religious themes, and those who are looking for a chance to alter religion begin changing it, it will lose its initial and original form, undergoing harmful and dangerous distortion.
Revelation, which is like pure and crystal clear water, will gradually lose this attribute as it flows through the minds of others. This gleaming light will weaken as it passes through the dark glass windows of the human mind. To make it short, as people add and omit things from religion, a point will be reached in which the main doctrines of religion will no longer be distinguishable.
Having said that, isn’t it necessary for someone to safeguard true Islamic teachings that are meant to remain forever, from alteration, resulting in religion maintaining its original form and being passed on to future generations? As we all know, the door of revelation has been closed for good and religion will never be revealed unto anyone till the end of time, so how is this religion to be preserved without undergoing any change or any superstitions being mixed with it? The only option is for an infallible individual to bear this responsibility, openly or secretly (as Imam Ali (as) said).
True religion, the way it was revealed unto Prophet Muhammad (pbuh), lies in the chest of the imam, serving as an unbreakable and secure safe, preserving this religion and preventing it from eradication, so that "so that the reasons proving the reality of truth as preached by religion and as demonstrated by His Prophet may not totally disappear".
d) During his occultation, the imam is like the sun behind the clouds. Clearly, being behind the clouds doesn’t mean that the sun no longer has any benefit or others can't benefit from it. One of the great benefits of such is that some are able to ascend to the sun itself, surpassing all the clouds concealing it, and make direct use, gradually reaching higher levels as a result.
e) Spiritual influence: (takwini training through takwini authority or wilayah) Imam Mahdi (aj) is a matchless and unequalled character whose strong rays of guidance effect those who have the capacity for such, thus being guided further [guidance has various degrees; some are guided from kufr to Islam, while others who are qualified as a result of good actions and piety, are guided from low levels of spirituality to higher ones; what is meant here is the latter], even though sometimes they aren’t aware of what has or is taking place.
The imam has authority and wilayah over our actions, meaning that he knows and sees all that has to do with the subject of guidance (which are the hearts and actions of people). Thus, all good and bad deeds, and the way to salvation and damnation go through him. So, the rank of imamate is linked with guidance, and what is meant by guidance here isn’t that he merely shows the way, what is meant is that he helps us reach the destination by actually "taking our hand", because showing the straight path is something that all prophets and believers have in common [while the imams are higher than all of them and possess more powers and options].
f) The purpose of creation: This world is like a green thriving garden; we the people being its bushes and trees, the fruitful ones being those in the path of perfection, and its weeds being those who have strayed from that path. Watering the garden is for the sake of the fruitful trees, not the weeds, "ان الارض یرثها عبادى الصالحون". If a day ever comes where there are no trees left in the garden and all of them die out, there will be no reason for watering the garden anymore, and to put it another way, if a day ever comes when the world bears no virtuous and pious individuals, there will no longer be any reason for Allah (swt) to continue bestowing his grace upon its inhabitants and will cease to grant them existence anymore. Having said this, the imam is the perfect individual living in our world today, the symbol of virtue and righteousness and the main purpose of creation, thus he is the one being addressed by Allah (swt) when He says: "لولاک لما خلقت الافلاک" [If it wasn’t for you, I wouldn’t have created the heavens (universe)], or when hadiths say: "لو بقیت الارض بغیر الامام لساخت" [If the world was to be without an imam, it would certainly cease to exist], and "بیمنه رزق الورى و بوجوده تثبت الارض و السماء" [It is by virtue of him [the imam] that the people are given sustenance and it is by his existence that the heavens and earth remain where they are [and don’t perish].
g) The mediator and link between Allah's (swt) grace and us: In Irfan (mysticism), it has been said that from the perspective of essential manifestations, the Divinity is a dominant unity, while in multiple active manifestations, He manifests specific determinations, yet this unity and multiplicity are both real, therefore there is a need for a third manifestation that can bear both and be a mediator between both realms of lordship and servitude. A third that is both linked with the realm above and our world. It is the infallibles only who bear both of these attributes, thus they are the only ones who can be the channels of Allah’s (swt) mercy to us. Therefore, when it is said that: “بهم یرزقون اللَّه عباده و بهم ینزل القطر من السماء و بهم تخرج برکات الارض”, it isn't exaggeration and is totally true. It is because of this rank that Imam Baqir (as) says: “نحن الاسماء الحسنى” [We are the great names of Allah (swt)].
Therefore, all of creation, bestowing grace upon all, guidance, training, spiritual development and… all are contingent on there being a live imam.
 Biharul-Anwar, vol. 52, pg. 91.
 Biharul-Anwar, vol. 51, pg. 152.
 Ithbatul-Hidayah, vol.6, pg. 437; see: Ebrahim Amini, Dadgostare Jahan, pp. 146-149.
 See: Cheshm be Rahe Mahdi, a group of writers of Majalleye Howzeh, pp. 345-347.
 Biharul-Anwar, vol. 51, pg. 113.
 Biharul-Anwar, vol. 52, pg. 122.
 See: Makarem Shirazi, Hukumate Jahaniye Mahdi, pp. 99-101.
 Tafsir Burhan, under verse 105 of surah Tawbah; RayShahri, Al-Qiyadah fil-Islam, pp. 84-85.
 See: Hukumate Jahaniye Mahdi, pp. 101-113.
 See: Different tafsirs of the Quran, under verse 106 of surah Tawbah; Al-Mizan, vol. 9, pg. 385; Tafsir Burhan, vol. 2, pg. 158; Usul Kafi, vol.1, pp. 219-220.
 "اللهم بلى لاتخلوا الارض من قائم للّله بحجة اما ظاهراً مشهوراً او خائفاً مغموراً لئلا تبطل حجج اللَّه و بیناته" Nahjul-Balaghah (qisarul-hikam (short sayings of wisdom)), pg. 147; Mizanul-Hikmah, vol. 1, pg. 167. (See: Usul Kafi, vol.1, pp. 178-180)
 It can be concluded from sermon 146 that Imam Mahdi is striving to protect Islam. See: Naser Makarem Shirazi, Hukumate Jahaniye Mahdi, pp. 226-229.
 Biharul-Anwar (old print), vol.13, pg. 129.
 Al-Mizan, vol.1, pp.275-276; Shieh dar Islam (Shia), pg.256, sixth chapter of chapters on Ma'rifatul-Imam. For further information about the impact of the imam's takwini authority on the guidance of the people, see: RayShahri, Al-Qiyadah fil-Islam, pp. 74-87. After citing a hadith from Usul Kafi, vol.1, hadiths 1, pg.194, the author says: "In addition to giving light, the sun behind the clouds also has an effect on the perfection and growth of different things in this world, the same goes with the "spiritual sun" of this world; he is the one who takes the hand of those who are ready and qualified and leads them to their destination of spiritual perfection. Al-Qiyadah fil-Islam, pg. 80.
 Hukumate Jahaniye Mahdi,pp. 268-269.
 Al-Kafi, vol. 1, pg. 179; Mizanul-Hikmah, vol.1, pg. 168.
 Hukumate Jahaniye Mahdi, pp. 268-269.
 Biharul-Anwar, vol. 23, pg. 19.
 Nurul-Thaqalain, vol. 2, pg. 103; Usul-Kafi, kitab Tawhid (the chapter on tawhid), bab nawadir (the title that says nawadir), hadith 4.