Yes God does know without our prayers. There are a few reasons why I pray. Here are some verses to help explain
Ephesians 3:20Now to him who is able to do immeasurably more than all we ask or imagine, according to his power that is at work within us,
God wants to use us- by praying we are lifting up to God what is on our heart and he will answer our prayer. He may bring help and safety directly to those in need or he might call us to give our time/money to help others who are there to bring about that help.
Ephesians 6:18And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people
God tells us to pray for his people. He tells us to pray about anything big or small, personal or affecting those far away.
Philippians 4:6Do not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present your requests to God. 7And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.
I pray because it brings me peace. It is very natural to talk about the things that worry us. Praying is sometimes about me lifting these things to God and me letting go of control of them. Trusting them into God's hands.