I create as a means of exploration.
There are things in my head—characters, vistas, cultures, metaphors, puns—that will never see the light of day otherwise, and often in the process of bringing them out, other things are discovered. That's why I am so interested in realism, in the Old Masters, in the Baroque, in fantasy & science fiction, even in typography. It's the ability to tell a story with an image, a phrase, a typeface; to convey more than what's there; to encourage the viewer/reader/audience to ponder, to bring their own stories to the piece.
And, of course, it's to create a pretty picture, to tell a riveting story. I've heard it said that you should draw what you want to see. Often that's the only way to experience something like that...no one can write the story you're dying to read, not the way you want it; no one can show you a picture of what's in your head. They can get close, but no further.
That is, I guess, the hardest thing about being creative: when the image in your head doesn't match what comes out on the page. But that's why we keep practicing, right?