Forms of prayer are necessary to guard us against the irreverence of wandering thoughts. If we pray without set words (read or remembered), our minds will stray from the subject; other thoughts will cross us, and we shall pursue them; we shall lose sight of His presence whom we are addressing. This wandering of mind is in good measure prevented, under God's blessing, by Forms of prayer. Thus a chief use of them is that of fixing the attention.
--Blessed John Henry Newman, “Forms of Private Prayer”, Parochial and Plain Sermons, Volume 1.