Here it is: the promised manifesto. Fanfiction is, among other things; important/cool/culturally valuable/sexy.
I find a lot of other people tend to expand the typical boundaries of fanfiction while defending it. "Dante's Inferno is just Bible fanfiction" etc. While yes, arguments can be made by Tumblr users and various Reddit threads that the Divine Comedy is basically self-insert, we're not here to have a conversation about definitions. Fanfiction is fundamentally derivative - "But all art is inspired by something!" - I hear you cry. Yes, but there's a difference between writing your own novel about a school for wizards, and "What if Harry Potter was a girl?". We don't need to pretend that these are comparable to defend fanfiction.
I don't even want to make the argument that fanfiction is well-written, or high quality. Of course, some of my favourites rival published works, but some, really don't. With the most amount of love in the world, fanfiction is community of amateurs, producing largely amateur quality work. And it's still important! Perhaps some of the most impactful works have been badly written: "Hi my name is Ebony Dark’ness Dementia Raven Way and I have long ebony black hair (that’s how I got my name) with purple streaks and red tips that reaches my mid-back and icy blue eyes like limpid tears and a lot of people tell me I look like Amy Lee (AN: if u don’t know who she is get da hell out of here!)."
Now, I'd like to apologise for sounding critical. I truly love fanfiction, it's been a part of my life since before I even knew about it. Somewhere buried in my parent's house is a notebook containing Warrior Cats self-insert fanfiction written by my 7 year old self. I've journeyed through LiveJournal, FanFiction.Net and Wattpad, and perhaps thousands of hours of formative years have been spent hiding under my duvet scrolling AO3 instead of sleeping. I have several fanfictions downloaded onto my kindle that I reread over and over again. I've created the most beautiful spreadsheet in the world (I love her and will take every opportunity to mention it) to record every fanfiction I've ever read. It has added so much joy (and angst and hurt/comfort) to my life and I hope it will continue to do so forever.
The most important thing about fanfiction is that is created with love. Someone found a piece of media, and loved it so much that there wasn't enough. Someone reached the end, with so much love to spare that they created more. Even a fanfiction that proposes "Hey canon sucks, but what if it was good?" is motivated by the love for the potential of media. Humanities base urge to share stories ability to empathize with fictional characters is what gives life to fanfiction.
Fanfiction is so important/cool/culturally valuable/sexy because love is. That's all there is to it. Sorry to get sentimental about it, love is just the point of everything.