It begs the question. So, if God is absolutely sovereign, why do anything? It's called fatalism. If fate (in this case, God) determines all that happens, then all that happens is determined and we don't need to do anything at all. What will be will be. This argument is thrown against the doctrine of Election. "If God chooses whom He will save, then why evangelize? God chooses whether or not you do anything." It is used to demonstrate that, in fact, God is not sovereign because if He is, everything is determined and we do nothing. And to the sincere believer who sees the inescapable conclusion that the Bible definitely teaches this kind of Sovereignty of God, it makes you wonder, "Why pray?" The Bible isn't very helpful on that last question. Jesus said, "Your Father knows what you need before you ask Him." (Matt 6:8) Precisely! So ... why pray?
As it turns out, just as there are multiple Scriptures that teach us that God is absolutely sovereign, there are multiple reasons to embrace His sovereignty and to pray (and evangelize and work and ...). Now, first, consider. Prayer consists of many components. There is praise and adoration. Surely the biblical certainty that God is Sovereign does not serve to limit our praise and adoration. So, pray. There is thanksgiving. Again, His Sovereignty should encourage, not defeat thanksgiving. So, pray. There is confession. Nothing about God's Sovereignty would negate our need to confess our sins to God. So, pray. That only leaves one other basic component -- supplication. Prayer is not merely making requests, but it includes it. So already we have a vast amount of praying we can do without even asking the question, "If God is Sovereign, why pray?" Still, down at this last piece, we might still ask, "Why pray?" Here are some good reasons.
We are, first and foremost, Christians. Oh, we might be some denomination of Christian, but we are, first, followers of Christ. If we are to follow Christ we ought to do what Christ did. And one thing Jesus repeatedly did was He prayed (Luke 11:1; Matt 19:13; Luke 9:28; Luke 5:16; Luke 6:12; John 17:1-26; Matt 26:39-44, etc.). Why pray? If Jesus, the Son of God, prayed, then so ought we to pray.
Clear in Scripture is the command to pray. Jesus taught His disciples, "Pray in this way." (Matt 6:9) He gave parables regarding the requirement to pray (Luke 18:1). Paul said, "Pray without ceasing." (1 Thess 5:17) We are told not to be worried, but rather "in every thing by prayer and supplication with thanksgiving let your requests be made known unto God" (Phil 4:6). (Note: there is a side benefit listed here. By making our requests known to a Sovereign Lord, we are told that "the peace of God, which surpasses all comprehension, will guard your hearts and your minds in Christ Jesus." (Phil 4:7) Thus, we are commanded and it is to our own benefit.) We are commanded to pray at all times for all men (1 Tim 2:1). Prayer is a required act of obedience. Why pray? Because God says so.
Not only are we commanded, we are invited. Jesus said, "Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you." (Matt 7:7) He told His disciples, "Whatever you ask in My name, that will I do, so that the Father may be glorified in the Son. If you ask Me anything in My name, I will do it." (John 14:13-14) James says, "You do not have because you do not ask." (James 4:2) and assures us that "The effective prayer of a righteous man can accomplish much." (James 5:16) Why pray? Because God invites us to.
God is sovereign -- absolutely -- but God is also a God of means. That is, He uses things to accomplish His purposes. More specifically, He uses people to accomplish His purposes. So He tells us to make disciples (Matt 28:19-20) and preach the Gospel (Mark 16:15), not because He lacks the ability to do it on His own, but because He uses means -- us. Prayer, then, as a matter of command and invitation, offers us an opportunity to participate with God in what He is doing. Thus, He says, "If you abide in Me, and My words abide in you, ask whatever you wish, and it will be done for you." (John 15:7) Pray in God's will and you can be certain that He will do it. Prayer is part of our participation in His divine work. So, pray.
For God's glory
When Jesus said, "Ask Me anything in My name," He wasn't talking about a magic formula. He was talking about asking under His authority for His glory. We are to "do all to the glory of God" (1 Cor 10:31). Praying is one of those things, and praying is at the outset for the glory of God. We ask; He works. He gets the glory. Definitely, pray.
To know God
Just before the famous "All things work together for good" verse, we read this.
In the same way the Spirit also helps our weakness; for we do not know how to pray as we should, but the Spirit Himself intercedes for us with groanings too deep for words; and He who searches the hearts knows what the mind of the Spirit is, because He intercedes for the saints according to the will of God. (Rom 8:26-27)Prayer is not merely "what's on my mind". Prayer is an exercise between pray-er and God via the Holy Spirit. It is an intimacy where the Spirit searches our hearts and expresses what we don't even know how to express to God. Prayer is our opportunity to bring our concerns to God, even though He knows them. In this sense it is for our benefit. In order to interface with God -- to know Him and have Him know you -- pray.
Prayer, at its basic level, is not mere communication. It is communion. It is communion with God. His Sovereignty only enhances that. In fact, only a person who rejects Man's Ultimate Self-Determination and embraces God's Absolute Sovereignty can pray safely for the lost because it is only a Sovereign God who can open a sin-dead heart to respond to the things of God (Acts 16:14). We don't know what we really need and we don't know what is really best (Rom 8:26-27), so it is only in a Sovereign God that we can have any real hope of praying for those things. In this sense, prayer is the ultimate expression of the total dependence of sinful human beings on a Sovereign Lord. In so doing, we obey Him and join Him in accomplishing His will, finally bringing Him glory. I think those are good reasons to both embrace a biblical view that God is Absolutely Sovereign and to pray to this Sovereign God.