Starting with a definition, fanfiction is when a fan of a particular popular media (books, shows, movies, more recently celebrities themselves, etc.) writes a story that writes a different version of the established canon. There are several sorts of receptions to the idea of fanfiction, and it is largely frowned upon in society, and as a writer, I want to tell people that this needs to stop. And here's why:
Fanfiction is just as important to the writing community as E-Journals or blogs are in a several ways. It provides a supporting structure and established world for a writer to practice character building, as well as provides for challenges in fitting into a world, fulfilling plot points, and establishing character dynamics. There is also a 'stress-free' environment for a writer to practice their craft by writing about something they really enjoy.
In creating this safe environment for a writer to practice, and challenge themselves in their craft, we are given practice with critical thinking, problem-solving, and more all within a provided structure. This helps to learn what an author is good at, what they need to work on, as well as emulate authors that they enjoy.
Another reason for this is that fanfiction is everywhere. We see it on TV show adaptions that vary from books, from book series like "The Shadowhunter Pages," and "Fifty Shades of Gray." Some of these even get fanfiction of their own, which adds to the cycle of creativity and brings more and more stories.
Yes, this will add to the argument that there are no original ideas in the world. However, all stories follow a basic arch, there are only so many stories to tell.
The difference is how we get from point A to point B, and what elements of craft get you there. Writing in any medium is important, and if starting with your fanfiction is how you get your name out there, then all the power to you, my friends. Because at the end of the day, you are writing for you, and people should be honored that you are sharing it, to begin with.