It is sometimes difficult for saints who believe that God is completely sovereign in all things to reconcile why we need to pray. We know that our prayers won't change the unchangeable, eternal decrees of God but yet Christ taught us to pray and said it would result in good things for us. To a point we know that there will always be aspects of God's sovereignty and our responsibility that we won't be able to grasp on this side of glory but I think there are some biblical reasons why we should pray that we can wrap our minds around. Let me suggests just four:
1. Much like faith in salvation; prayer is the means God has chosen to accomplish some of his will. While he has ordained all things to come to pass, he has ordained many of those things to come to pass through the prayers of the saints. Which means if we don't pray they won't happen. By this I do not mean that God depends on our obedience but I believe he will move his people to pray for certain things before he does them and if he doesn't move them, he isn't going to do it. Likewise, if a person doesn't believe he will not be saved; we don't have to worry whether he was elect or not because God saves when he moves men to call upon him for salvation.
2. Another reason we should pray is because it keep us involved in the Lord's work and helps keep us from apathy. When people fail to see this they easily fall into hyper-Calvinism. "God doesn't need me to witness or anything else or I might rob him of glory. He can do it all by himself". While there is truth here it is not full truth. What results are churches and people who are content to sit by and coldly watch people die and go to a Christless eternity. This is why some prayers are petitions. God has graciously and wisely given us the opportunity to pray for each other which in turn gives us cause to exercise our love to each other which is the Second Commandment. It helps us from being totally consumed with ourselves which is one of our great tendencies anyway.
3. It gives us opportunities to praise God in our hearts and with our churches as we tell of answered prayer. It is one thing to praise him for his wisdom in providence but when he answers prayers poured out by burdened hearts this gives us the opportunity to give him heart felt, loving praise that would not come apart from the opportunity to pray.
4. Lastly it helps us grow in the grace of God and mature as saints. When Paul tells us to rejoice always and be anxious for nothing in Philippians 4 he follows it up by telling us to pray for everything by supplications and thanksgiving. I think it would be hard to rejoice always and trust fully on him without the invitation, ability and opportunity to lay everything at his sovereign feet. In fact right in the middle of this passage he says, "Let your reasonableness be known to everyone. The Lord is at hand". I think this is not so much a statement that the Lord is about to come back but that he is with us ready to help us in every situation.
If God is going to work in our churches it will not be apart from a burdened, praying people. He will not bless people who don't care enough about his work and glory to ask him to bless. If we don't care enough to ask, he won't bless because he does it to be glorified, not taken for granted.