"If God already knows why should we pray?" is a common question asked by those struggling with the tension between God's sovereignty and man's responsibility.
I love the clarity of Christ's statements concerning this issue. Recently I've been jolted by a small portion of Matthew 6:8-9.
"...For your Father knows what you need before you ask Him. Pray then..."
The fact that God knows our needs before we ask is precicely the motive Christ highlighted to stimulate us to pray. There is no tension in Christ's mind!