The Power of Prayer of Agreement

Matthew 18:19-20

"Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I among them.”

If, then, God already knows our needs, why do we pray to Him?
“God does not want our wishes to be made know to Himself, since He cannot be ignorant of them; but He wants our desire to be exercised in prayer, thus enabling  us to grasp what He is preparing to give. That is something very great indeed; but we are small and limited vessels for the receiving of it”
(St Augustine)