I used to write and read Jurassic Park fanfiction. I can’t say whether what I wrote back then was good or bad, but writing it and being read was important for me as a growing writer. The readership was small, but readers gave useful notes on what did/didn’t work in my writing. I got better with each new piece of fanfiction. I made friends with other writers on the site (some of whom I remain friends with today) and it was always cool to impress the writers we admired most on the site with a new chapter or short story. It was like a very specific sort of writer’s group. And sure, most of the feedback rarely went beyond a ‘good job!’ comment but still, it was something. Writing can be lonely but writer friends can help with that. In writing, we’re all students, trying to learn the craft. A fanfiction writer either looks at their writing as a fun hobby or as the minor leagues for the next step. And there’s nothing wrong with either point of view. Both are useful. Both are healthy for a creative mind.
While there’s typically no money to be made in fanfiction, that’s not always the case. Tie-in novelizations are big money bestsellers (especially Star Wars) and the popular trend of legacy sequels basically have young filmmakers making big budget fanfilms based on the movies that were popular when they were young (see: Creed, Blade Runner 2049, and Star Wars: The Force Awakens). Obviously, writing fanfiction online is not the doorway to writing fanfiction for a living — there must be some in-between success that gets you that gig — but I raise the point because while we may not read fanfiction online we do enjoy its cousins in the bookstores and multiplexes.
The main point I wish to make is: don’t shit on fanfiction. The readers enjoy it and the writers are learning their craft. And isn’t that what we’re all hoping to do with our writing anyway — to entertain others and to improve our abilities?