Transfer of Knowledge
In creating a metaphor, the writer or speaker is drawing on a source of information that the reader or listener is already familiar with in order to enhance his understanding of a target idea. For example, in stating, "My aunt is a bit of a porcupine," a speaker would not expect someone to infer that her aunt is actually part rodent. Instead, she would be drawing on the writer's understanding of porcupines as prickly, lonesome creatures to inform his understanding of her aunt's character.
Metaphors are generally compact, vivid statements with an ability to make a clear connection between otherwise inexplicable or abstract qualities. These characteristics make it possible for writers and speakers to convey a large amount of information to their audiences in a creative, yet concise, way.