According to Sankara there is no creation. There is only vivarta vada - apparent manifestation.

Swami Vivekananda says (Complete Works, V3, pp 12-14, also available here under the heading Lectures and Discourses, sub-heading The Free Soul - http://cwsv.belurmath.org/volume_3/vol_3_frame.htm):

Before going into the practical part, we will take up one more intellectual question. So far the logic is tremendously rigorous. If man reasons, there is no place for him to stand until he comes to this, that there is but One Existence, that everything else is nothing. There is no other way left for rational mankind but to take this view. But how is it that what is infinite, ever perfect, ever blessed, Existence-Knowledge-Bliss Absolute, has come under these delusions? It is the same question that has been asked all the world over. In the vulgar form the question becomes, "How did sin come into this world?" This is the most vulgar and sensuous form of the question, and the other is the most philosophic form, but the answer is the same. The same question has been asked in various grades and fashions, but in its lower forms it finds no solution, because the stories of apples and serpents and women do not give the explanation. In that state, the question is childish, and so is the answer. But the question has assumed very high proportions now: "How did this illusion come?" And the answer is as fine. The answer is that we cannot expect any answer to an impossible question. The very question is impossible in terms. You have no right to ask that question. Why? What is perfection? That which is beyond time, space, and causation — that is perfect. Then you ask how the perfect became imperfect. In logical language the question may be put in this form: "How did that which is beyond causation become caused?" You contradict yourself. You first admit it is beyond causation, and then ask what causes it. This question can only be asked within the limits of causation. As far as time and space and causation extend, so far can this question be asked. But beyond that it will be nonsense to ask it, because the question is illogical. Within time, space, and causation it can never be answered, and what answer may lie beyond these limits can only be known when we have transcended them; therefore the wise will let this question rest. When a man is ill, he devotes himself to curing his disease without insisting that he must first learn how he came to have it.

There is another form of this question, a little lower, but more practical and illustrative: What produced this delusion? Can any reality produce delusion? Certainly not. We see that one delusion produces another, and so on. It is delusion always that produces delusion. It is disease that produces disease, and not health that produces disease. The wave is the same thing as the water, the effect is the cause in another form. The effect is delusion, and therefore the cause must be delusion. What produced this delusion? Another delusion. And so on without beginning. The only question that remains for you to ask is: Does not this break your monism, because you get two existences in the universe, one yourself and the other the delusion? The answer is: Delusion cannot be called an existence. Thousands of dreams come into your life, but do not form any part of your life. Dreams come and go; they have no existence. To call delusion existence will be sophistry. Therefore there is only one individual existence in the universe, ever free, and ever blessed; and that is what you are. This is the last conclusion reached by the Advaitists.

SO to answer your question more directly, there is no 'need' for creation. Why did Brahma want to become many? Do you mean Brahman or Brahma? If Brahman, the Upanishads simply say that he became many. The Taittiriya Upanishad (3.1) says:

That from which these beings are born, by which they live after birth and into which they they enter at death--try to know That. That is Brahman.

And the Aitareya Upanishad (1.1.1-2) says:

This universe, my dear, was but the Real (Sat) in the beginning--One only without a second [ekamevadvitiyam]. It thought, 'may I be many, (the Atman) willed, "Let me project world!' So It projected these worlds.

Your next question is not logical. You ask - what will happen if nothing will exist in this universe? If the universe exists, then something exists - the universe implies existence, if not of anything else, of the 5 subtle elements.

Iswara is simply the Nirguna Brahman seen through the lens of maya. Our brains are too small and must think in terms of name and form. We cannot conceive of That which is beyond name and form, so we give the Unmanifest a name and form so that we can try to comprehend It.

What is the relation between the Brahman, the Devas, and human? It is all One simply appearing as many. The devas and humans are all jivas that are caught in the meshes of maya. It is all Brahman. The Chandogya Upanishad says (3.14.1):

All this is verily Brahman.