Mary Gonzaga Haessly in her seminal, though virtually unobtainable, work on Rhetoric in the Sunday Collects of the Roman Missal:

 

Liturgical prayer is a form of public speech, and hence it

is not surprising that in Christian antiquity, the threefold

officia of classical rhetoric were applied to it as well. The

reasons for this are presented succinctly by Mary Gonzaga

Haessly in her seminal, though virtually unobtainable, work

on Rhetoric in the Sunday Collects of the Roman Missal:

All these devices of the art of language are necessary for us, for

they enable us: (r) to grasp clearly the lessons embodied in the

Prayers (docere); (2) to make these lessons more acceptable to

us through the charm of diction and structure, in a word,

through their appeal to our aesthetic sense (delectare); (3) to

persuade us (movere) to mold our conduct in accordance with

the principles of faith set forth in the Prayers. This explains

why rhetoric is, and must be, found in the liturgy: it is to dis-

pose us to pray "ut oportet," as we ought to pray.'