People believe for the same reason that the scientists conducts experiments for the same reason the writer writes, the musician plays, the artist paints, the stargazer stares into the night's sky, and so on and so forth. They want the truth.
We look around and try to make sense of our world, explain why things happen the way they do. For the ancient Hebrews, they really didn't have the time or desire to worry about how things came into being; hence why they said "Well, God did it in seven days. It just makes sense." It wasn't the entire fact by fact truth, but it was a good answer (for the time). Its the same thing today when people say "Well, the universe started with the Big Bang. It just makes sense." It might not be the entire fact by fact truth, but it's a good answer for our time. Maybe in the future there will be better answers, theories, ideas. In reality, science and religion aren't as different as people make them out to be. They both, at their very core, long to understand our world.
The problem really comes when people distort that, on both sides (such as 'God needing your money' that someone referenced a couple posts ago). Trying to use science to prove or disprove religious belief is like trying to use religion to prove or disprove scientific thought, both of which have ironically been done throughout the ages. Honestly, it's downright silly. It seems as though the bigger question here is whether or not God actually exists. This, ironically as well, is not tied to anyone's belief one way or another (unless you have this wierd notion of God where our belief gives him strength or something...but that's a whole 'nother bag of grief)
I really think these science vs. religion debates are pointless, and not because they're opposites, but because their end goals really are (ideally) the same. They just examine it through a different human faculty: science primarily through the mind, and religion primarily through the spirit, the anima, the soul, the something that makes us different from everything else out there. Don't fault the fact that humans are incredibly flawed and screwed both up to discount their merits.
And by the way, Darwin's writings and experiments actually reinforced his belief in God. He was in absolute awe at the complexity of life and how it all managed to work out (specifically the human eye). Whether that is true or not, still, I don't think we can prove, but can everyone please stop making it out like Darwin is the athiestic messiah or something, it's not true. Athiesm was around long before Darwin, and it had plenty of arguments before him as well.
Also, another interesting question, because it seems to come up here in terms of belief: What is proof? Is it objective or subjective? Do you need proof to believe something, and if you don't, is it then ok to have that belief?