Disclaimer: I do not tell anybody what they must believe and I won't fight about religion with anybody. I do not believe that everybody's spiritual journey can be or will be like anybody else's. So having said that, and since you asked (
):
I do believe we have been given the power, by God, to alter our own destiny. I believe God knows the outcome if we continue on the path we are on, and via prayer, which for all practical purposes is the means by which we communicate with God, we can hear and obey to make better choices or stop something we are doing that is harmful to ourselves or others which is my definition of sin. If there was no way to be better, to choose more wisely, to make a difference, then we are simply characters on some sort of cosmic stage helpless to alter or avoid a fate already determined for us. And the God I know is not cruel.
I have to believe prayer does make a difference or I can see no purpose for it. Certainly the ancient people of the Old Testament believed that things could be different because the Old Testament is full of examples of it. And there are many New Testament examples and throughout history where an encounter with the Divine changed a person's life in amazing ways.
Will sometimes we experience a miracle? I believe I have. Do most prayers result in obvious miracles? Mine sure don't. But all I know is I am convinced that I am far better off with prayer than when I neglect it.