Coming into this thread rather late, sorry....
Why write it? Pretty much all my completed fiction, fan and otherwise, has been of the won't-let-me-sleep-otherwise variety... I tend to come up with ideas while trying to sleep, and often have to scribble them on afuton-side scrap of paper to have a chance of sleep.
I've never written fiction for an audience, but it rounds off the experience to get affirmation from friends, and it's a real thrill getting praise from strangers.
Tavia