Assuming its people who have a religious belief who do the praying, then it gives them the sense of doing something useful, whether it works or not doesn't matter, it helps the prayee.
I don't really think we can knock people who pray, if someone says they are praying for you, good for them, they are thinking of you, and that can't be bad.
As for World peace, the only time that;ll happen, is when the Human species has been totally wiped out, then the prayers will have been answered.