"Why pray if God is going to do what He wants anyway?"
"We need to trust God completely, in obedience, knowing that prayer changes things."
There are some things that we will not understand until we get to heaven. On the one hand, we know that the sovereignty of God is very clear in the Scripture. Paul says in Ephesians 1:11 that God "works out everything in conformity with the purpose of his will, . . ." On the other hand, God's Word says that He responds to the prayers of the faithful ones (see 1 Peter 3:11-12; James 5:16). And so, while those things may seem in tension with one another, they are not.
We need to trust God completely, in obedience, knowing that prayer changes things. God said so. He promised so. He has given us prayer as an opportunity to cry out to Him to see a change in our culture and our society—and God will answer our prayer. Trust Him as the sovereign God, and pray in obedience.