I think God serves two basic purposes. First, it helps to answer the question 'Why?', which many of you have pointed out is beginning to become a moot point thanks to scientific knowledge.
The second purpose is to provide hope. People without hope don't survive/thrive/strive for anything better. They either get by while waiting for something to happen, or they cease to live(generally through their own hands)
I think children learn to trust a benevolent being (typically parent)early on to help them do things they cannot do themselves, to watch over them and keep them safe, and this can translate to trusting in a similar entity (God,etc.) as they grow physically independent.
When I as an atheist look for help, I look to people I trust and care for, ancillary to looking to myself. I think theists tend to do the exact same thing, but may have a far better attitude about it because they can go to their god first, and as we all know, God works through others
'How do you know god doesn't exist' is the same as saying 'how do you know Santa Claus doesn't exist'
It had a purpose, at one point, but that need is gone, at least for me, but I'm glad anyone that needs God, has it. Just respect that it ain't my bitch.