I bet you are reading this right now and thinking, "really? You're going to to try and say fanfiction, that thing that people do for free and fun in their spare time, holds some important aspect within creative mediums?"
While fanfiction can seem strange and baffling at times, it stands as an important medium that helps teach young fans how to express their own form of creativity by applying it to something they are familiar with.
When a person takes the time to write fanfiction about a certain television show, comic book, book, movie and/or video game, it should be safe to say that that person has a deep understanding of the characters and world of that particular work of art (the "fan" in fan fiction). Fans want to express what they love about the characters, story and world of a work they love and add their own personal creative touches to it, whether it be an expression of love for a pair of characters or how these characters would act and behave in AUs (Alternative Universe) such as modern-day or medieval times.
Fanfiction acts as a sort of creative play, similar to a young child playing with a random assortment of toys and making up a story and character moments around the toys they are given. This becomes even more evident when the subject of crossover fanfiction between two or more separate properties is brought up. A person is made to think how characters like Merida from "Brave" and Hiccup from "How To Train Your Dragon" would interact or how the crew of the Starship Enterprise would react to meeting a Jedi for the first time.
While this might seem strange and absurd at times, it lays the fundamental groundwork for creative thinking and creative synthesis. It allows creative people to explore the endless possibilities that come from the world of fiction. How would the Marvel heroes and DC heroes interact with each other? Would Tony Stark and Bruce Wayne have something in common with both being billionaires who fight crime or would Bruce despise having to work with Tony and his narcissism?
These are the kinds of questions fanfiction writers and readers ask themselves constantly. These kinds of writers (with the proper amount of skill) could become great writers in their own right and could be very skilled in the realm of adaptation (since that's basically what fan fiction is). In fact, I myself am an avid fanfiction reader and hope to one day write a few stories myself (when I actually get the time, which is very rare).
Fanfiction, while goofy at times, acts as an early outlet for young people to express their ideas and emotions. They use what they already know about a certain work of art and add to it or twist it in a unique and unexpected way. You might think that fan fiction isn't that creative due to it being based on preexisting material and that coming up with your own original idea is a superior expression of creativity. While originality is important just imagine what the world would be like if someone like George Lucas hadn't looked at all the fantasy stories he loved as a kid and wondered what it would be like if those stories were set in a science fiction universe.