Fanfiction and the people who write it have received a bad reputation over the years, as people who aren't creative enough to write their own work, who are stealing from others (they aren't, Fanfiction.net requires you to put a disclaimer in your work stating that the characters are not your own) and who essentially just want to write soft porn.
As a former fanfiction writer, I take issue with this and today I'm going to talk about why fanfiction isn't all bad and, in fact, can be a good thing.
1. It helps improve your writing
The key thing with writing is practice, practice, practice. Fanfiction gives writers who may not have the ideas for their own original work an outlet in which to practice their writing within a given framework - you already have the characters and world, all you have to do is write, and this for me was so helpful when I was younger. It also provides you with instant feedback for your work, as fanfic is a community of writers.
2. It adds diversity to existing canon
Fanfic writers have been racebending, gender-bending and including characters of different sexual and gender orientations for years. Fanfic allows authors to include diversity into what otherwise might be a very allocishet story, and in my opinion that can only be a good thing.
3. All media is a form of fanfiction in one way or another
A lot of existing media is inspired by other stories, so in a sense, it is a form of fanfiction. Think 'Bridget Jones' Diary': it's a retelling of Pride and Prejudice, essentially a form of fanfic. The hit TV show 'Once Upon A Time' takes fairytales and adapts them, so in a way it is another kind of fanfic. Even BBC's 'Sherlock' is in a way a type of fanfic, after all it takes Arthur Conan Doyle's characters and puts them in a modern setting.
4. It allows you to write for an audience
The one drawback to writing your own fiction is that it's difficult to get instant feedback on it. People are busy, and if you do get other people to read your work then it will take a while for them to get back to you, so for the most part you are the main audience for that work. Fanfiction provides you with an inbuilt audience who can give you instant feedback to improve your writing, and who are already invested in the characters and world you are exploring.
5. It's fun
There's no point in doing something creative if you don't enjoy it, and, like it or lump it, writing fanfic is incredibly fun. It's an easy and safe way to practice your writing and it doesn't matter if you suck because the other writers on fanfiction websites are lovely and will help you get better. It allows you to connect with other writers in the same fandom and get to create something new with the characters you love. What's so bad about that? In My opinion: nothing at all.