This is a tough one to write.
You see, as a writer who totally isn't one who wrote a universe that sparks fanfiction, it's really easy to detest it. Especially when you focus on all the bad ones written out there by teenage girls fantasizing about being in that universe. I remember admitting I've been there before when I first wrote about my previous DA:O crush.
I know, I'm such a girl.
Anyway, fanfiction with Mary-Sues and that are poorly written definitely make fan-fiction seem like a horrible idea. Sometimes, it is. But sometimes, it's a fantastic tool for writing.
Here me out, please.
When you've graduated enough in your writing talent, you realize there are essential parts to every character, every single one, that help you write the scene. Motivations, goals, flaws, histories, yada yada yada. What's even more important is that you know how to utilize those components to make a complex character that is relatable. The same goes for setting--different components, but the same basic idea of using the tool effectively.
Writing a fanfiction, you already have that set out in front of you, which is one of the hardest parts of writing any story. You don't have to flesh out as much, and you've been given a lot of details to work with. This gives you some flexibility to hone your craft. You now have something that helps you, as a writer that is, work on delivery and your use of the (hopefully extensive) vocabulary and style you've come across over the years.
It comes to being a great way to practice. People in all other sorts of professions practice somehow, why not writers?
I'm sure some of you are figuring out I'm only writing this entire thing to defend any fanfiction I may decide to write soon. Believe me, I am not posting it here. As much as I can justify its existence, I still feel a little shame when I come across the idea.
Not as much shame as reading my original series Star Trek fanfiction from when I was in highschool. *shudder*