If God has a divine plan, what's the point of prayer?

Think about it. If you're praying for a relative's disease to go away, but it doesn't go away, then you'll probably say it just wasn't part of God's plan for your relative to get better. But if your relative does get better, then it means it was part of God's plan. So what's the point of praying if God's plan will inevitably happen anyway? Isn't it rather selfish to pray if your endeavour is to make God change his divine plan of the universe to accommodate your desires?