I should actually rephrase what I said above. I don't write fanfic in the sense most people people use it as - about shows or books I like. I do, however, write a fair bit of flavor material for the science fiction 4X
games I play, consisting of in-character AARs
and various bits of background fluff along the lines of what's been written for Sword of the Stars
(said game's fluff being the main reason I play it, actually - I'm sort of 'meh' on a lot of the mechanics, but it's incredibly
immersive). I've never written any fanfiction in the sense of a story or such, but I would say this counts.
As for why I do this... well, partially it's because, even in those games that have campaigns or other continuing stories, it's still in the background enough that I don't feel like I'm stealing other people's work like I would with "normal" fanfiction. And partially, it's because there tends to be just enough Backstory
that I have something to work off of, but not enough that I have to disregard some canonical material for my own additions. Basically, though, I find this sort of thing fun to do.