The real answer is that he doesn't need us to do anything for him, but it's part of a testing process. He created us with free will, and wants to see if we're worthy of going to live with him when we die.
We can only do this by choosing to do his will. This could be anything from wearing a funny hat on Tuesdays to only eating fish while standing on one leg to killing people we think he might not approve of. It's not clear though - you pretty much have to guess what he wants because there are various different versions of it that no-one can agree on.
It's all very strange and mysterious but that's the way he rolls.