In a way that’s a form of a bigger question that sometimes people who ask that question don’t ask, which is, “If God has planned all things, why should we do anything?” And we understand that what we do as Christians we do for a variety of reasons.
One is because God has taught us to do it in His Word. So the bargain basement level answer to the question is that we pray because God has told us to pray, even if we don’t understand how prayer works and how He employs our prayer.
The second thing is to say that the sovereignty of God does not destroy human responsibility but is actually the foundation for it. And the sovereignty of God never destroys secondary causes. So what we want to say is that God plans all things. We pray because, among other things, our prayers are part of the instrumentation God has planned in order that He would fulfill His purposes.
When you think about it, it is really a wonderfully gracious thing that God has the power to do things without us—without our prayers, without our intercession—but He’s our Father, and so He wants to catch us up into His purposes just like a father would do with his own children. You could say, “Get out of my way, I’ll do that,” but a true human father doesn’t do that. A true human father comes along and brings his children along, and brings them into his purposes, and delights to see them grow in fulfilling his aspirations for them.