Why should I pray if God already knows what will happen?
Here's the answer:
God works out His perfect plan through prayer.
God's plan is His will—what He wants to happen. God's will always happens because He is all-powerful. God's will is always best because He is all-wise and all-good.
So if God's plan always happens, why should we pray and ask God for things? Won't He do His own will whether we ask Him or not?
We pray because God tells us to pray (1 Thessalonians 5:17). If we disobey God's command to pray, God will still do His plan. But then we may not know He worked, and we won't be able to thank Him and trust Him more.
God also uses prayer to work His perfect plan. Prayer is like a hammer. A hammer can't hit a nail into a board on its own. But a person uses a hammer to pound the nail into the board. God uses prayer to work His perfect plan. He doesn't have to use prayer, but He chooses to use prayer.
Jesus taught us to pray that God's will be done (Matthew 6:10). Before Jesus was arrested, He prayed that if it were possible, God wouldn't make Jesus bear the punishment for sinners by dying on the cross. But then He prayed, "But let what you want be done, not what I want" (Matthew 26:39c). So we should pray for God's will, even knowing that God's plan will happen.
"The LORD who rules over all has taken an oath. He has said, 'You can be sure that what I have planned will happen. What I have decided will take place'" (Isaiah 14:24).
"There is one thing we can be sure of when we come to God in prayer. If we ask anything in keeping with what he wants, he hears us" (1 John 5:14).