What's the point?

What purpose is supposed to be served by praying? I mean, I genuinely don't understand it, from a logical perspective. God is supposed to be omniscient, right? That means he already knows what you want, and how badly you want it, and whether or not you deserve to get it. What possible difference can it make to ask him for it? Do you think you're going to make him aware of some aspect of your wish that he had previously overlooked?