When we make specific requests in our prayers, this is not so as to inform God, for He already knows our hearts; we make them so that we may be brought to contrition. We also do it because we desire to remain longer in His presence, attentively addressing yet more words to Him, giving thanks to Him, acknowledging the many blessings we have received from Him, for as long as we can, as St John Chrysostom says of the Prophet David….[it] is done out of longing and so that the word of divine Scripture should be imprinted in the intellect of whoever is reading or praying.
For God knows all things…we, however, have need of hearing things, so that we may know what we ask for and why we are praying, and may be filled with gratitude and cleave to God through our entreaties. It is through such repetition that we avoid being overcome by our enemies when we are troubled in thought, for then they will not find us unmindful of Him; and it is also through it that, helped by prayer and the study of divine Scripture, we may come to acquire the virtues about which the holy fathers have written in their various works, through the grace of the Holy Spirit.
~St Peter of Damaskos
Prayerful Life 