Metaphors can enhance writing in many ways, which is why so many writers use them. Even the most amatuer writers use metaphors without realizing it.
The first reason to use metaphors is to give readers a picture in their minds of what is being discussed. If an author is trying to get a reader to see how beautiful the subject's eyes are, he or she might use the metaphor, "her eyes were a sparkling pond." The comparison of eyes to a pond helps the reader see just how beautiful the eyes are.
Another reason to use metaphors is to help readers understand a concept. Extended metaphors are especially useful for this. An extended metaphor is a metaphor that lasts for multiple lines and has symbols that keep the metaphor going. Consider Sylvia Plath's poem, "Metaphors," as an example of this. In the poem, Plath discusses what it feels like to be a pregnant woman. The speaker never comes out and says she is pregnant, but she uses symbols and metaphors throughout to show that she is. She compares herself to an "elephant, a ponderous house, /A melon strolling on two tendrils..." and later "a stage, a cow in calf." Through the use of metaphors, Plath is able to give her readers a deeper understanding of what it felt to be pregnant without actually having to say "I feel huge."
Metaphors can also be used to simply make the writing more interesting. If a writer uses literal language, especially in fiction or literary non-fiction, the writing can be boring and dry. Coming up with original metaphors helps to make the writing more interesting while, as stated above, helping the reader to understand what the author is trying to convey.
Several reasons come to mind as to the purpose of metaphors in writing.
First metaphors are a great ways to exaggerate an emotion is your reader's mind. Metaphors create great pictures in the mind of readers and if crafted carefully, readers can dwell upon metaphors with their imagination.
Metaphors infuse emotion into text, especially nonfiction. Matter-of-fact nonfiction texts use metaphors to disguise facts and cause the reader to emotionally react to the material.
Metaphors are a great change in style. Used moderately and skillfully metaphors can create a great sense of flow to a story. They can easily convey the feelings of a character and are great sentence stretchers.