Similes and metaphors help writers explain and describe objects, people or situations. Sometimes the best way to give expository detail is by comparing an object of study to something else. For example, describing the color of a shirt as "blue like the sky" offers more precision of detail than simply calling it "blue." This enables the reader to visualize the shirt using a frame of reference with which he is already familiar. In this vein, simile and metaphor are appropriate for use in most kinds of writing.
Especially, but not exclusively, in creative works, writers use similes and metaphors to inject emotion into the description of a person, object or situation. Instead of saying, for example, "I felt sad," you could write, "I felt like an Olympic athlete who had just lost the most important race of his life." In addition to adding detail, the lavish simile provides an overflow of emotion that the reader can feel. Similarly, a metaphor such as "a river of tears" indicates an excess of emotion.
Similes and metaphors are an effective way to make the language in your writing more exhilarating. In addition to offering detail and emotion, similes and metaphors add life to the structure of the prose itself. For example, the sentence, "The dog barked" is flat on its own. Make the language sing by adding metaphorical flourishes. For example, "The dark barked a thousand claps of thunder as if the gates to the underworld were suddenly thrown open."
You can include similes and metaphors in your writing by using familiar, everyday figures of speech that draw on these rhetorical devices. Figures of speech offer a convenient shorthand for illustrating a larger idea. For example, you could write, "All hands were on deck for the family dinner preparation." The sailing metaphor does not add description or emotion, but uses a common and accepted turn of speech to signify the fact that everyone pitched in to help.