Stephen King, in his book 'On writing' (and other writers), says one should put some gap of time between drafts. So for example, wait a month after finishing the 1st draft, before starting work on the 2nd draft.
Why is this needed? Is it needed?
I would say yes, you should leave a gap between the first draft and redraft, with the qualifier that you should do what you find works for yourself.
The reason many authors such as Stephen King suggest this is because it allows you to come back and read what you've written with "fresh" eyes. It is often the case that, when you've spent a long time working on a piece of writing, constantly going back and forth, you become so used to the text that you cannot see any possible way to improve it. This can lead to frustration and/or a false sense of perfection with what you've written. By giving it a break, you can come back almost as if you've "forgotten" it.
However, this may not be what works for you. I seem to recall Zadie Smith once saying that, when she was writing White Teeth, she would spend hours every day re-reading everything that she had previously written before she would carry on, editing along the way.
I am just beginning to understand the nuances of re-writing and what i have found is if you have been reading your writing recently, you don't re-read it as much as you are recalling what you read.. That is to say, even though you move your eyes through the document once again, you are not just recalling what you wrote(and a biased version of what you wrote) rather than really re-reading and trying to see how you can improve. I have found that for me it's not a question of a time gap as much as what i do in the gap between. i.e: If i go away and read something else and come back am less fresh compared to say i go away do my day job(programming) and come back... the passive nature of reading other stuff seems to keep me dwelling somehow.. it seems.. Extending that hypothesis, i would say , going for a run/gym/physical activity should improve the freshness even more.