The purpose is to make the narrative more mentally stimulating, and thus, more enjoyable and memorable.
The use of sound repetitions (even mentally) and "plays on words" are usually interesting to humans. Sometimes we enjoy them in common discussions (some people are very good at puns), but usually it takes more thought than we bring to everyday chats.
However, in literature the author has both the time to construct some interesting phrases and the necessity of doing so in order to hold the reader's (or an audience's) attention. It makes the experience more memorable and more enjoyable.
Even "contrived" prose in plays or literature is usually more interesting to a person. If plays and stories were written only in the mundane phrases commonly used in speech, they would receive very little attention. They would be thought of primarily as boring.
Excellent oration does draw on this fact, but oration can be understood as another form of performance rather than a normal conversation.