It's (often) well written!
Sure, not everyone can write and there are no editors or standards that have to be met for fan fiction. I'm sure there's still poorly written stories floating around the internet of Batman and the Joker going at it thanks to a young me who really should have researched anal first (or at least known enough to include lube!). But with such a large selection of fan fiction out there, it's easy to weed through the bad stuff and get right to the hot detailed action.
Most people who write fan fiction do so because they're fascinated with the lives and personalities of the characters involved. Instead of corny plots with shallow, laughable, dialogue, you can really get inside the character's heads and get attached. Does she feel guilty about sleeping with the boss? Is he worried that she might quit her job the next morning or think less of him? Has she had a crush on him for years and can hardly believe that he returns the sentiment? Not only will you be wondering those things, but you'll find your answers too!
It's free!
Of course, that's a good thing because you're probably not rich. But that's also a good thing because it's pure. Fan fiction writers aren't rushing to meet a deadline, or coming up with something because they need that paycheck. They're writing because they're passionate about writing and passionate about the series and, as someone who's read plenty of published erotica and plenty of fan fiction, I can tell you that it really shows.
The fact that they're writing for themselves and not for a paycheck also leaves a lot of room for experimentation. There's fan fiction for any strange kink you can think of, with no consideration wasted on whether or not something so obscure would sell well. There's no requirements for characters to be conventionally attractive, either, or for sex to be flawless. For fiction, it can be surprisingly real.
It's something you're actually interested in!
Have you spent years with a crush on Jessica Rabbit, or wondering what Kirk and Spock do in between missions? Maybe not, but it's more likely that you've been wondering about that than about whether or not Unnamed Busty Female #1 is going to screw the pizza delivery man. There's a fic out there for everyone, all it takes is some digging!