Why would an author use metaphors

to activate the sense of the reader and create a specific mood

Explanation:

excerpt from "the   daffodils" by william wordsworth

figurative language is a resource that fiction writers use to make better descriptions of what they want to write about or express. they can improve their stories, as they can elaborate more vivid and colorful sentences. this way, the reader will be able to have more detailed images in his/her mind.

places, objects, people and even feelings or emotions can be described with a lot of detail to metaphors, similes and personifications. making different kinds of comparisons to make a description more specific, as well as assigning human qualities to non-human objects (and vice-versa) can be very useful for science fiction writers.  

on the other hand, alliterations allow writers to elaborate more entertaining and creative sentences. it consists in using a sequence of words that start with the same consonant letter. they can be used to emphasize something important the writer wants to express, or just as a fun literary device.