This answer is from Bhagavad Gita perspective.
What is the need of creation? Why Brahma wants to become many?
Neither need nor any reason to create the universe.
Also Brahman (or Brahma) should be interpreted as in one-ness, whether it is one or many.
BG 9.8 - Keeping "My" nature (prakruti) under control, "I" create [& destroy] the world of beings again & again; which are helpless under the nature (prakruti)
BG 9.9 - And O Arjuna, those actions (Karma-s) don't bind Me, as I am uninterested & detached to them
BG 9.10 - Under "My" supervision, the Prakruti produces moving & non-moving [beings]; With[out] this purpose the world repeats
What will happen if nothing will exist in this universe? Will it affect Iswara?
Indeed the nothingness (or ShunyatA) is the ultimate reality, which many refer as supreme God or Iswara. Hence, we can deduce that there is nothing really existed, exists & will exist.
BG 8.20 - But different from that "unmanifested", is the another eternal "unmanifest" reality, which is not destroyed when all beings are destroyed.
BG 13.13 - I shall speak of that which is knowable. Knowing which, one attains immortality. The supreme Brahman devoted to 'Me', is beginning-less. 'That' is said to be neither existent nor non-existent. (existent = being)
Like, when we are dreaming, so much time passes and things also look real. Only upon waking up, we find that, less time has passed & it was an imagination. Similarly, whatever is manifesting currently for us, looks "real" to us, can be interpreted as being imagined.
This is also the core theme of movie Inception.
BG 9.7 — O son of Kunti, all elementary world becomes "My" nature at end of cycle(Kalpa); Again "I" create them at the beginning of the cycle.
Is there any relation between existence of Supreme Brahma[n], Devas and humans?
The supreme Brahman or "Me" or whatever is the ultimate reality -- is not perceivable as discussed above. Which is depicted as "Nirguna" (beyond formless). The other aspect is "Saguna" (formful or formless). That is the cause of everything existent. It keeps happening as part of Eternal Return:
- "Everything" comes into being out of "Nothing"
- All beings keep mutating; This existence remains for "X" duration
- "Everything" vanishes into "Nothing"; This non-existence remains for "X" duration
BG 8.18 - On arrival of day, all manifestations originate from "Unmanifest"; On arrival of night they annihilate into [what is] known as "Unmanifest" only.
BG 8.19 - This [same] elementary world only happens again & again; Annihilates upon arrival of night, [and] originates upon arrival of day.
Mainly Brahman, Deva-s & humans have a relation of time, i.e. "X" duration.
If a consciousness (Purusha) is at the level of:
- "Me" / Atma, then X + X = 0 (no time)
- Brahman, then X + X = 1 Day + 1 Night of Brahman
- Deva, then X + X = several days of Deva
- Pitru, then X + X = several years of Pitru
- Humans, then X + X = several eras of Humans
There can be intermediate planes as well. Refer the section Q-2 in this answer for the scripture proof.
BTW, after humans, there are further lower planes of existence: like animals, insects, bacteria, ... Lower the plane, more the time is spent to realise the supreme reality. Once reached at lowest, again there is a sense of "no time" (i.e. "Me").
If a person has more Sattva mode (goodness), it journeys towards "Me" by going to higher regions than humans; with more Tamas mode (ignorance), one journeys towards "Me" by going to lower regions than humans. The mode of Rajas (passion/desire) only binds to any particular plane.
BG 14.18 - Those with sattva go higher [regions]; those with rajas stay in the middle; those with tamas, having despicable actions, go [further] down.