Answers
If God already knows, why pray at all?
Because prayer was never primarily a way of informing God. It is the relationship in which He has chosen to act — He ordains the asking as well as the answer, the way a good father genuinely gives because a child genuinely asked. Jesus, who had nothing to learn from anyone, prayed all night. If it were only about transferring information, that would be inexplicable.
Go and see for yourself
Be not ye therefore like unto them: for your Father knoweth what things ye have need of, before ye ask him.
Matthew 6:8
Ye lust, and have not: ye kill, and desire to have, and cannot obtain: ye fight and war, yet ye have not, because ye ask not.
James 4:2
And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God.
Luke 6:12