If God already has a plan, do my prayers really matter?
Prayer matters because God's foreknowledge includes every request you will ever make. He knows ahead of time what you will ask, so He can answer prayers while still fitting them into His sovereign will. You cannot change a sovereign God's mind, but He has planned everything according to the fact that you will pray.
Les explains that in the foreknowledge of God, He knows everything that is going to be prayed for. Because everything is based on His foreknowledge, He can answer prayers and it will still fit in His sovereign will, since He knows ahead of time that you are going to ask for it. Les says it becomes part and parcel of His will, but He could not have done it had you not asked, because then His foreknowledge would have said, they are not going to ask for it anyway. So as we pray, we do not change God's mind; you cannot change a sovereign God's mind, but He has already planned everything according to the fact that you are going to pray for it. Les also notes that in this verse all three of God's answers are evident, and that when we pray we should consider what kind of answer we expect. He illustrates this with a child asking a parent for something, expecting yes or no, and with a young lad asking for a bicycle who was told, when you get a little older, meaning later.