In response, they are told that other than Ali (as), none of the other imams ever came to power and weren’t able to accomplish what the Shia expect their imams to! So why do they make false claims about their imams?! If they think a bit they will realize that according to their own beliefs, those who they think to be their imams actually aren't, because none of the objectives they have been sent for have been accomplished.
Allah’s (swt) wisdom is what calls for Him to appoint imams after the final prophet (pbuh) in order to carry on his responsibilities so that the main goal of sending all previous prophets and the perfection of man remain possible. It is because of this that the Shia believe that the imams are directly appointed by Allah (swt) and that all of the prophet’s (pbuh) responsibilities (other than receiving revelation) rest on their shoulders; governing the Muslim nation being one of them. Therefore, the existence of the imam itself is a grace, regardless of whether he ever possesses political power or not.
Our Sunni brothers see the imams the same as they see other normal rulers and kings and don’t believe they have a rank that differs from theirs. It is because of this that they think the major role the imam plays is to manage the people’s everyday worldly affairs, resulting in them saying that if an imam never comes to power, his being an imam is pointless and isn't considered a grace from Allah (swt). But according to the Shia, the imam is one to take the place of the prophet (pbuh) in everything other than receiving revelation, as both Shias and Sunnis narrate the Prophet Muhammad (pbuh) addressing Imam Ali (as) saying: “You compared to me are like Prophet Harun (pbuh) compared to Prophet Musa (pbuh), the only difference being that no prophet and messenger is to come after me.” On this basis, Imam Ali (as) was to be the first imam after the prophet (pbuh), followed by eleven other imams, who were to succeed the prophet (pbuh) in every rank and responsibility he had. It is also because of this same reason that the imam must be an infallible individual and invulnerable to any form of sin and even mistake, and since identifying the infallible individual is beyond man’s capability, he needs to be directly appointed by Allah (swt), because although after the prophet’s (pbuh) death, revelation ceased, but all of his other main and very important responsibilities were still there; responsibilities such as being a reference for the people on religious matters, explaining Islamic rulings and laws, explaining Quranic verses, removing religious doubts and answering religious questions, training the people, enjoining good and forbidding evil at its highest and presenting consultation to the Muslims, being a refuge for those seeking it, leading the people, judging between them, etc. Yes, we also agree that one of the responsibilities of the imam is the leadership of the Muslim nation, but it isn't the main reason for having an imam. It is totally baseless to say that being an imam means to have political power, so if a person doesn’t possess it, you can be sure he isn't an imam. This is the reason why the Shia believe imamate to be one of the pillars of Islam and consider there being an imam a grace from Allah (swt) that He must fulfill for the people. Government and political power are very small branches of imamate[, not its roots].
It is because all of these pivotal duties that the existence of the imam is grace and entails many blessings for society and the people, even if he never comes to power.
One of the important things an imam living amongst the people does is that he plays an important role in them having the tendency to live righteous lives and stay away from sin and badness. This is also considered grace.
Why is that? It is because they are all infallible, while all other than them aren't and might be influenced by their personal desires and cross the boundaries of humanity and commit sins; they are always in need of others to help them stay away from straying from the straight path.
For further information, see: The Logical Reasoning for Imamate, Question 614 (site: 671).