Why God should ask us to pray, when God knows what we need before we ask, may perplex us if we do not realize that our Lord and God does not want to know what we want (for God cannot fail to know it) but wants us rather to exercise our desire through our prayers, so that we may be able to receive what God is preparing to give us. God's gift is very great indeed, but our capacity is too small and limited to receive it. That is why we are told: Enlarge your desires, do not bear the yoke with unbelievers.
The deeper our faith, the stronger our hope, the greater our desire, the larger will be our capacity to receive that gift, which is very great indeed.....In this faith, hope and love we pray always with unwearied desire. Howeve, at set times and seasons we also pray to God in words, so that by these signs we may instruct ourselves and mark the progress we have made in our desire, and spur ourselves on to deepen it. The more fervent the desire, the more worthy will be its fruit. When the Apostle tells us: Pray without ceasing, he means this: Desire unceasingly that life of happiness which is nothing if not eternal, and ask it of the one who is able to give it.
From a letter to Proba by St. Augustine