To borrow a line from Caedmon's Call:
"You can't plan the ends, but not plan the means."
God needs not our help. Just as it is with evangelism. We do it because we love Him. Those who love Him obey His commands. We do it because we understand, according to Scripture, God has means by which He accomplishes His ends.
We also pray because we're confident that what we ask, so long as it is according to His good, pleasing, and perfect will, will be accomplished...since it will be ordained by the Almighty Sovereign God of Scripture Who is not bound by anything other than His own promises. Those promises have our good and His glory in mind.
Prayer, In my humble opinion, is also another way of molding the believer.
Everydody else already said it better, but I thought I'd just ramble my