Words of wisdom from James Enge:
In an imaginary world, everything is there for a reason. The reason may be sheer inertia–the ground is made of soil, because it didn’t occur to the writer to make it anything else. But a shrewd writer doesn’t make those choices via inertia. He makes the ground into an angry vegetable that devours random people at the dark of the twelfth moon. She makes it into a vast expanse of shining incorruptible metal. They make it into something on purpose, to make an impact on the reader. Stuff that in realistic fiction would be corny–instances of the pathetic fallacy–are part of the basic toolkit for shrewd writers of sf/f. The whole world can be a metaphor in imaginative fiction.
Or not. Part of the impact of the metaphor requires the writer to take the material in the imaginary world at face value, as real for the purposes of the story–like a comedian keeping a straight face while telling a joke. I’ll sleep on this and try to figure out if it makes any sense.