After all, doesn’t God know best? Does he need advice from us on how to run the world? Lewis says you could make the same argument against any activity, not just prayer:
“Why wash your hands? If God intends them to be clean, they’ll come clean without your washing them… Why ask for the salt? Why put on your boots? Why do anything?”
Yet God has chosen a style of governing that allows people to take part. We can cut down trees and dam rivers, cultivate the land, hurt or help each other, rebel against our creator, and kill the prophets.
“Prayer as a means of advancing God’s kingdom is no stranger than any other means. Go into all the nations and preach the gospel, Jesus told his disciples, thus launching the missionary movement with all its harrowing history. …Heal the sick, visit prisoners, feed the hungry, house strangers… Consistently, God chooses the course of actions in which human partners can contribute most.”
Prayer: Does It Make a Difference? By Philip Yancey, pp. 136-7. The Lewis quote is from God in the Dock, 1980, p. 104-5.