From the  Purdue OWL:

Metaphor:

• A comparison between two things, based on resemblance or similarity, without using “like” or “as.”
• The act of giving a thing a name that belongs to something else.
• The transferring of things and words from their proper signification to an improper similitude for the sake of beauty, necessity, polish, or emphasis.
• A device for seeing something in terms of something else.
• Understanding and experiencing one thing in terms of another
• A simile contracted to its smallest dimensions

Metaphors enliven ordinary language, they encourage interpretation, they give maximum meaning with a minimum of words, and they create new meanings: Metaphor allows you to write about feelings, thoughts, things, or experiences for which there are no easy words. Metaphors are necessary.