As someone who enjoys reading GOOD fanfiction and writes fanfiction (although at this point I haven't written much of it due to having an original idea that can actually, umm... survive in this brain for a long while, but I'm digressing here), I have to agree with a simple thing.
A lot of it is pure, absolute shit.
But that's what makes finding the GOOD stuff all the more enjoyable. As for why we want to read it instead of reading more original stuff by "Real Authors"?
1.) We like the characters to the point where we want MORE adventures with the characters than canon is going to give us anytime soon. Like say, Dragon Ball which has been completed for years now or Harry Potter, especially when there were YEARS between the books. There's only so many times one can re-read/rewatch the actual canon before getting bored of it while still wanting more of the characters.
2.) It takes effort to go to the library, while fanfiction is available via the computer. Not really a good defense of fanfiction, but hey.
Now as for all of the people who are like "Why don't they just create their own characters and world?" I have the following to say.
1. Again, we love the characters we already know and love and want to see more of them, but everyone else sucks too much and we want to prove them wrong (hopefully without sucking ourselves). In addition, we like trying to think of how our beloved familiar characters would react to situations that they never got into in canon.
2. Coming up with an original world and characters? Is a lot harder in some ways than using pre-created characters are, especially with STAYING INTERESTED IN WHAT YOU'RE WRITING. Which is why so many of the terrible fanfic authors don't do it apparently.
3. Go check fictionpress.net, fanfiction.net's sister site, only for original fiction. Oh, what's that... most of the stuff there is shit too? It only seems like original fiction is higher quality than fanfiction by concept because most of the shitty original novels never get published because people are paid to burn their eyes out while finding the good stuff to try to make money off of.
4. As my previous point first demonstrated, sometimes we fanfic authors write BOTH fanfic and original fic. Actually, I kinda said that in my opening paragraph anyway.