One more reason to write fanfic: because it allows you to write the sort of story that needs to be written as an incident occurring in an existing universe that the reader is familiar with. Sometimes all the exposition needed to set up the payoff of the story blunts the impact of the story. When the reader can be assumed to already have a large chunk of background, you can strip the story down to its bare essentials and punch the reader in the gut. Or you can play with the expectations set up by the pre-existing knowledge, and surprise the reader by giving them what they *didn't* expect.

This is what draws me to fanfic to begin with. After you've read the entire cannon in your particular fandom, there's a desire for something more. Since my fandom is comics, there's a desire for some adult conversations, a little realism, and a lot of what they're doing when not pummelling one another into paste.

Fanfic writers don't need to offer a 40 year summary of what the characters have been doing since their creation. They assume the reader is familiar and jump right into the meat of the story. As a reader, I find the well told fanfic stories incredibly satisfying.