SNM:
First of all, to get myself off. For me, turning a fantasy into prose, expanding it and adding details and turning the people in it into detailed characters, is much more gratifying than just having it in my head.
I will take a fantasy of mine or even a life experience and message it, expand it, reinvent it and refine it. I enjoy reading my stories several months after I have written it. Writing forces you to deal with characters, personalities and your own feelings. After outlining a story I have looked people up I haven't seen in years to get their reflections on events I remember.