Most people have a deepseated need to justify their existence.

Why am I here? What is the purpose of life?

God makes them feel special about it. I'm here because a magical man in the sky wants me to be here. Most people are happier believing in God than they would be if they were ever convinced that he wasn't there. Even if God is a fabrication of the human mind, he fills his purpose. His purpose is to give people hope, to make them accept their lot in life and run with it. Maybe he makes them not commit crime because he's always lookin over their shoulder. Maybe he makes them live a good life so they don't go to hell.

At some basic level, I think most people progress from the belief that there _should be_ a God to the belief in any specific man-in-the-sky. I think that's telling.