In the world of existence, some things are a cause with specific effects on other things, like water (the cause) to quench the thirst (the effect) of a human being, and we can say that there exists an existential correlation between a cause and its effect. So, when we come to investigate the reality of the existential role of the Imam, we need to first investigate this correlation between causes and their effects, and to differentiate between ‘mentally posited’ (subjective) concepts and ‘existential (objective) ones.
‘Mentally posited’, or subjective, concepts like political leadership, vary with time, place, and human factors like the type of government and political systems in place, since it is the human population that decides who to elect and how. Populations can elect and overthrow a political leader/party/regime at a time (and method) of their choosing. The same applies to the concept of (subjective) ownership of consumer items like cars and private property, which can be transferred from one owner to the next with no limitations. There is no existential connection between the people and their leader, or a person and his house like that between a person and his eye-sight or the ability to move his/her arms. We are able to transplant body parts, but not the senses which utilize these body parts in order to function.
So the question is: has the infallible Imam been given a similar existential role in relation to the world of existence? If the answer is in the affirmative, and we are able to prove this existential dimension to the personality of the Imam, then it becomes self-evident that the political responsibility undertaken by the Imam is secondary to his existential role.
The theological scholars of the Sunni school, however, reduced the Imam’s responsibility to a single dimension, political leadership in the earthly realm (as mentioned in previous posts). This was the cause for the ‘frustration’ with their Shi’a counterparts with respect to this on-going dispute (the concept of Imamah), and that it (Imamah defined as political leadership) is out-of-date and debating it is a pointless exercise. Since the (Twelver Shi’a) 12th Imam is in occultation and therefore plays no role in people’s lives, so what’s the point in believing in his existence or his occultation if he cannot rule as leader of the Islamic nation? (We have refuted this fallacy in previous posts, and the refutation should be further clarified once the existential dimension of the Imam is demonstrated).
The theory behind the existential role of the Imam
Before we delve into this fascinating theory, we require some introductory points:
1) Mankind was created in this world for a reason and has itself a goal:
وَمَا خَلَقْتُ الْجِنَّ وَالْإِنسَ إِلَّا لِيَعْبُدُونِ
And I have not created the jinn and the men except that they should serve Me. (Al Dhariyat:56)
which is to become servants of God through acts of worship. However, this goal is an intermediary station to the final one, which is to reach a state of ‘certainty’:
وَاعْبُدْ رَبَّكَ حَتَّىٰ يَأْتِيَكَ الْيَقِينُ
And serve your Lord until there comes to you that which is certain. (Al Hijr:99)
This (certainty) is the most honourable goal a human being could attain, as is stated in the description of Prophet Ibrahim’s path to spiritual perfection:
وَكَذَٰلِكَ نُرِي إِبْرَاهِيمَ مَلَكُوتَ السَّمَاوَاتِ وَالْأَرْضِ وَلِيَكُونَ مِنَ الْمُوقِنِينَ
And thus did We show Ibrahim the kingdom of the heavens and the earth and that he might be of those who are certain.(Al An’am:75)
And the goal of certainty is to reach nearness to Allah (swt) and the return to Him:
أَفَحَسِبْتُمْ أَنَّمَا خَلَقْنَاكُمْ عَبَثًا وَأَنَّكُمْ إِلَيْنَا لَا تُرْجَعُونَ
What! did you then think that We had created you in vain and that you shall not be returned to Us? (Al Mu’minun:115)
يَا أَيُّهَا الْإِنسَانُ إِنَّكَ كَادِحٌ إِلَىٰ رَبِّكَ كَدْحًا فَمُلَاقِيهِ
O man! surely you must strive (to attain) to your Lord, a hard striving until you meet Him. (Al Inshiqaq:6)
2) Mankind is not able to identify what perfections it needs to seek and attain solely relying on its senses and intellect, for both its worldly and other-worldly happiness. Nor what the shortest way to reach these perfections could be. This is where the role of the divine religions comes into play, which show us which perfections to follow (certainty in their belief in God through reaching nearness to Him), and the quickest and shortest routes to attain them (‘the straight path’). And since mankind is unable to establish these two requirements by itself, it is in need of divine guidance, in its different forms:
a. The first is the innate disposition, which every human being is born with (fitrah):
فَأَقِمْ وَجْهَكَ لِلدِّينِ حَنِيفًا ۚ فِطْرَتَ اللَّـهِ الَّتِي فَطَرَ النَّاسَ عَلَيْهَا ۚ لَا تَبْدِيلَ لِخَلْقِ اللَّـهِ ۚ ذَٰلِكَ الدِّينُ الْقَيِّمُ وَلَـٰكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
Then set your face upright for religion in the right state- the nature (fitrah) made by Allah in which He has made men; there is no altering of Allah’s creation; that is the right religion, but most people do not know. (Al Rum:30)
and is a fixed divine reality:
سُنَّةَ اللَّـهِ فِي الَّذِينَ خَلَوْا مِن قَبْلُ ۖ وَلَن تَجِدَ لِسُنَّةِ اللَّـهِ تَبْدِيلًا
(Such has been) the course of Allah with respect to those who have gone before; and you shall not find any change in the course of Allah. (Al Ahzab:62)
فَلَن تَجِدَ لِسُنَّتِ اللَّـهِ تَبْدِيلًا ۖ وَلَن تَجِدَ لِسُنَّتِ اللَّـهِ تَحْوِيلًا
For you shall not find any alteration in the course of Allah; and you shall not find any change in the course of Allah. (Fatir:43)
However, this innate guidance does neither specify the perfections, nor the path to attain them. It gives mankind the desire and motivation to attain limitless degrees of virtues and knowledge.
b. The second is the legislative guidance, embodied by prophets and messengers carrying divine messages to show mankind the path towards their intended goals and happiness (nearness to God).
c. The third is existential guidance, which can be also defined to be ‘aiding the traveller towards his goal’, whereas the second type of guidance (showing the path), does not guarantee the traveller to reach his goal. This type of guidance does not allow for deviation, since there is a guarantee that the one guiding the individual will take him to his intended goal.
3) Everything has been created for mankind’s utilization:
اللَّـهُ الَّذِي خَلَقَ السَّمَاوَاتِ وَالْأَرْضَ وَأَنزَلَ مِنَ السَّمَاءِ مَاءً فَأَخْرَجَ بِهِ مِنَ الثَّمَرَاتِ رِزْقًا لَّكُمْ ۖ وَسَخَّرَ لَكُمُ الْفُلْكَ لِتَجْرِيَ فِي الْبَحْرِ بِأَمْرِهِ ۖ وَسَخَّرَ لَكُمُ الْأَنْهَارَ
Allah is He Who created the heavens and the earth and sent down water from the clouds, then brought forth with it fruits as a sustenance for you, and He has made the ships subservient to you, that they might run their course in the sea by His command, and He has made the rivers subservient to you.(Ibrahim:32)
وَسَخَّرَ لَكُم مَّا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ جَمِيعًا مِّنْهُ ۚ إِنَّ فِي ذَٰلِكَ لَآيَاتٍ لِّقَوْمٍ يَتَفَكَّرُونَ
And He has made subservient to you whatsoever is in the heavens and whatsoever is in the earth, all, from Himself; most surely there are signs in this for a people who reflect. (Al Jathiyah:13)
Some people think that this ‘gift to utilize’ is a privilege to humanity which they can exploit how they pleased. In fact, humanity’s abilities and intellect are the gifts given to them to utilise everything else that has been created in an intelligent and just manner. Therefore, the world is a responsibility on mankinds shoulders. We find in narrations that each part in our body has a right on us to treat it in the way God intended. So every blessing and gift given to us, has a corresponding responsibility attached to it. 
This is where the Quranic idea of ‘divine vicegerency’ becomes clear, since it means that mankind, as God’s representatives on Earth, have a divine responsibility towards all the things that have been created for them. Yet, they are able to refuse this responsibility as a result of their power of free will, and instead utilise these gifts in the path of evil and corruption instead of good and justice.
So, the Quranic system has promised the third type of (existential) guidance exclusively to the Imam. And the role of mankind is to follow in the footsteps of these 3 guidances towards divine closeness and its intended goal and perfection.
 Wasa’el Al Shi’a, Vol. 15, Book of Jihad of the soul, pg.172; or the Book of Rights, by Imam Zain-ul Abideen (as).