I like wux's answer, but here's an interesting idea ...

Let's answer that question with the "multiverse". Assuming there is a God, He has a plan and everything is absolute, then alternate realities can play a part. If every decision humans make creates another universe in which a different choice was taken, then praying can indeed have been factored in and can make a difference because there is an alternate reality where either God did not grant the wishes of the one praying or the one praying did not ask.