First things first: what is a rhetorical question? It is a question asked not with the hope of getting an answer, but with the hope of making a point or creating dramatic effect. The answer to a rhetorical question may be non-existent, it may be so obvious that it does not require an answer, or it may be immediately answered by the person who asked it.

In some cases, a rhetorical question can be used to affirm assent in a colloquial alternative to just saying "yes." For example, consider the following exchange, which includes one ordinary question and one rhetorical question.

"Would you like to go out for lunch today?"
"Sure, why not?"

The words "why not?" are technically a question, but they are not asking anything. This is an example of why rhetorical questions are used in holding casual conversation.

I would argue that one of the primary reasons that rhetorical questions are used in books and films is to create interest. For example, consider the title of the 1993 film What's Eating Gilbert Grape? This is a rhetorical question, and it piques interest in the story because people will naturally be curious as to what indeed has happened to be "eating" Gilbert Grape.

The best example I can think of relating to using rhetorical questions to make a point is the famous line from Romeo and Juliet, when Juliet says,

What's in a name? That which we call a rose by any other name would smell as sweet.

The fact that there is no pause after the questions tells us that she was not looking for an answer but was using a question to draw attention to the ridiculous feud between the Capulets and the Montagues.