In her post, Lily wrote about a literary agent who decreed that one should never have more than two metaphors or similes in an entire literary work. As you might expect, many of Lily's readers offered their opinions in the comments, disagreeing with the literary agent. Write without metaphor? You must be joking! I don't think I can converse without metaphor or simile.
I think writers use simile and metaphor because thinking up a good simile/metaphor is just plain fun. Wit, as I recall, has to do with combining dissimilar things, in a way that gives the reader (and the writer) pleasure.
I agree with Lily that metaphor works similarly to wit. And both are good fun. But, the writerly pleasure of crafting a great metaphor is the aftereffect of achiving one's purpose in writing: to communicate with your reader. If humor is the recognition of the intersection of two incongruent thoughts, certainly metaphor is also.
As I wrote in the comments on Lily's blog, I like to think of metaphor as like a Venn diagram. Each vector in the diagram is disparate, but where they overlap, in the unexpected, not previously explored, region is where the writer uses figurative language to clarify something for the reader in a new and exciting way.
Although seemingly a simplistic way to describe a complex thought process, I see it like this:
What I know and what you know may be different, but if I am trying to convey an idea to you, I should start on common ground. I take two things that you have knowledge of and link those concepts together so that you can understand what I want to convey. F. Scott Fitzgerald is credited with saying that "[T]he test of a first-rate intelligence is the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function." What a writer does with metaphor is to stretch that common ground in unexpected ways to broaden the reader's understanding by purposely forcing the reader to not only hold those two disparate ideas, but at the same time to merge them into a new concept.
The speaker of The Red Red Rose, to use Robert Burns' poem as an example, says "My love is like a red, red, rose". The reader thinks: "okay, she's like a flower. A pretty, red flower". The poet and the reader both understand love, and roses, and now they both understand that the speaker of the poem sees his love like a beautiful rose. The poem continues: "That's newly sprung in June". Now the reader understands more -- it is a fresh, just blossoming flower. But Burns doesn't stop there: O my love's like the melody/That's sweetly played in tune. Now the reader can move on to another image of the lover's sweetheart; his understanding is deepened by the second metaphor. The overlapping area of the diagram expands in yet another unexpected way.
But, you might say, the image of a lover as a red rose -- or any kind of rose, perhaps even a sickly one as Blake used (see "The Sick Rose") -- is cliche. And you would be right. But when it was first used, it was not cliche; it was unexpected. The same could be said for Homer's description of 'rosy-fingered dawn'. But, the lasting power of these images is due to their effectiveness.
But metaphor and simile aren't only used in poetry. Anytime something needs to be described, metaphor is the tool to have at hand. Take this passage from Howard Nemerov's essay "On Metaphor":
While I'm thinking about metaphor, a flock of purple finches arrives on the lawn. Since I haven't seen these birds for some years, I am only fairly sure of their being in fact purple finches, so I get down Peterson's Field Guide and read his description: "Male: About the size of House Sparrow, rosy-red, brightest on head and rump." That checks quite well, but his next remark -- "a sparrow dipped in raspberry juice" -- is decisive: it fits. I look out the window again and now I know that I am seeing purple finches." (A Howard Nemerov Reader, pp223)
I first read this essay of Nemerov's many years ago and I used it to describe metaphor to a classroom of bored teenagers. They may not have understood Burns or Blake on the whole, but they understood what Nemerov was writing about when he described his recognition of the birds dipped in raspberry juice.
Whether the metaphor is describing dawn, a beloved, a unfamiliar bird, or a pole dancer, each of these examples uses the common knowledge of the dissimilar to communicate precisely what the writer wants the reader to understand. Perhaps it is the cliched use of figurative language that the uncredited literary agent was referring to. I would agree that the cliched should be avoided. But using metaphor to convey an image by comparing unlike things that are known to the reader -- like comparing the exotic dancer to a rotisserie chicken -- is a tool that all writers use. It is basic to communication. That region in the Venn diagrams of our minds where two unlike ideas come together is where our limited vocabularies and mere words on the page are used to stretch our imaginations and to make something perfectly clear.