What is the logic behind prayer?

[john b. loved]

There is no logic in your logic. God gives men autonomy to live their life as they please, but will call all men to judgment. God tells us to pray because unless we express our desire before Him He will leave things as they are. It has nothing to do with changing the will of God, but affirming it.
Concerning free will, you are free as a citizen to do what you like unless it violates the law of the land, in which case you will pay the penalty. If you violate the command of God, how much more severe will be the punishment, and especially if you violate the rights of those who God loves ?

Now the reason it makes no sense to you is because you have no faith. You believe your own opinion more than the word of God, and the result is you are in darkness, and those in darkness cannot see where they came from or where they are going.

http://www.youtube.com/watch?v=G3KU-fvd6lc