why did He create us?​

It is obvious that Allah does not need us, so why did He create us?

The idea that need is set against vainty is absurd!

The need is set against wisdom and not vainty.

A rich doctor with a good reputation may treat people without needing anything from them, rather he treats them for their own sake. here we can not describe his deed as futile.

The wisdom and the great purpose behind the act are not in the circle of need / vanity

A swimmer may save a child out of having mercy on him and then he may leave him and go without waiting for any thanks from his parents. In this case we do not say that he saved him for a need or in vain, but he did out of his noble manners.

There is no correlation between need and vanity.

In Saheeh Muslim we read the Qudsi Hadeeth: " O My slaves, were the first of you, and the last of you, the human of you and the jinn of you to be as wicked as the most wicked heart of any man of you, that would not decrease My dominion in a thing. O My slaves, were the first of you and the last of you, the human of you and the jinn of you to stand in one place and make a request of Me, and were I to give everyone what he requested, that would not decrease what I have, any more than a needle decrease the sea if put into it." 

The Patient`s ignorance of the doctor`s wisdom does not mean that doctor`s decisions are futile.

The knowledge of the divine wisdom does not require understanding all the dimensions of wisdom, but it is enough to understand some of them.

It is sufficient to be aware that we are commanded to perform the acts of worship, to know their requirements and to know the existence of the divine wisdom. This is sufficient for us in the general terms, otherwise we will be like those who disbelieve in what they do not understand.

Allah is the Most wise and he created us for a purpose.