First read a previous reply I made on a similar subject:
So, God knows everything, yet we still pray, because:
– Jesus himself prayed
– Jesus asked us to pray and taught us to pray, and He is God. It’s good enough for God, it definitely is good enough for me.
– Jesus asked us to pray persistently, St. Paul said ceaselessly.
Prayer does not change God, it changes me. It is not that what I need does not come to me unless I pray, but rather God has a gift greater than my current capacity. He is not merely looking at my current needs, but my final goal of full holiness, and how far I still fall short. But He cannot give me all He wants to now because my soul and my desire are too narrow. I don’t really want all of Him yet, or to let go of all my sinfulness and attachments yet. So He asks us to pray, so we may learn to desire Him ultimately; and this process of desiring, has the wonderful effect of enlarging our heart and mind to Him. He therefore reserves certain extra graces to give us, each time we desire Him in prayer, so that the more our heart is stretched out as we desire Him, the more of Him and of His graces we are filled; and the more we are full of Him, the more we detach ourselves from our old self, and conform to His Will.