I know this is a question that I struggled through when I was first introduced to Reformed Theology. This is something that still comes up with both Christian and non-Christian friends alike. Why does it matter if pray? God's plan is perfect and sovereign. His perfect will is going to pass anyway, right?
Keller quotes Calvin stating:
"He so tempers the outcome of events according to his incomprehensible plan that the prayers of the saints, which are a mixture of faith and error, are not nullified."Keller expands on Calvin's points:
"If God's will is always right, and submission to it is so important, why pray for anything with fervor and confidence? Calvin lists the reasons. God invites us to do so and promises to answer prayers--because he is good and our loving heavenly Father. Also, God often waits to give a blessing until you have prayed for it. Why? Good things that we do not ask for will usually be interpreted by our hearts as the fruit of our own wisdom and diligence. Gifts from God that are not acknowledged as such are deadly to the soul, because they thicken the illusion of self-sufficiency that leads to overconfidence and sets us up for failure."
This should give us enormous confidence to pray! God ordains certain things to come to pass only through our prayers. Our prayers don't need to be perfect because they're not answered based on the quality that we put forth. Everything is funneled through Christ, he is the only channel in which our prayers make it to God. So pray with confidence! It's your job to pray. Sometimes they will be answered, other times not. But who cares because whatever the result is, "Thy will be done."