The use of a figurative language in a story to to get someone to understand a part of a theme, or using details to explain by using one of the figurative language. For example, if you are comparing two different things that are alike use metaphor.
As if improving the story, the use of a figurative language is to use a language that you would not write usually. This way when you start using a figurative language, you can talk about a part of the theme just by using one of its use. That's the benefits for a figurative language.