Fr. Thomas Hopko observes
Prayer is learned only by praying. No man can teach another man to pray. But a good way to begin to pray is to use the prayers of the Prayer Book. This is so because "not knowing for what to pray," the Holy Spirit reveals in the prayers of the saints the proper form and content of prayer. In the prayers of the books, especially the Lord's Prayer, we not only pray truly by putting ourselves into the words of the prayers, but we also learn for what we must pray.
Put differently, without this "school of the prayer" found in the prayerbooks, our prayer may remain at a certain level. When we recite these prayers, we start to move towards the saints' level in our prayers and, ulimately, in the whole of our lives.