"Generalisations are always stupid-"
Hmm, does that include this one?
Here's my four cents (adjusted for inflation). I write FanFic for several reasons. I happen to like the concept of Star Trek more than most recent content, so by writing, I can experience what I'd like to see in the shows/movies. It also is good writing practice to make onself be able to at the same time conform to certain external standards, while also generating a readible, original story. It's also free from possible future commercialization, so it is really writing for writing's sake.
As to why I read the stuff. It's fun. I like classics too, but don't choose to limit myself to preconceived ideas about any genre or sub-genra of literature/writing.