When I wrote fanfic, I actually did it to provide a more realism-oriented take on characters or situations. I added legal immigration details to the Heroes world and showed a slash relationship somewhat averted in the world of sports circa 2010. For me, fanfic was a way to work on developing a more realism-centric style without needing to juggle a whole lot of elements (such as original character development or new plot arcs). Nowadays, I have drifted from fanfic because it's gotten easier to balance all of those elements.