Memorizing other's work is the best exercise; improves memory, gives ready examples to be imitated (sense of Quintilian 2.7).
There is no place in a speech where confusion of memory or loss of fluency is more shaming; a faulty Prooemium is like a badly scarred face, and it is a bad pilot indeed who runs his ship aground while leaving the harbor (Quintilian 4.1.61).
SD XI.17 Holy Spirit wants to work through the Word heard and committed to memory