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!
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 embarrassing to admit I read it, but I find fanfic to often be superior to porn. I've been attracted to explicit erotic fiction since I was a precocious young tween. Fiction is more immersive for me than watching something. And if you dig, there are plenty of fanfic writers out there just as good as published authors. You can have great plots with your raunchy explicit porn. Of course, there is pretty awful writing out there too, and a billion "mary sues". And some fandoms seem to attract better writers than others. One nice thing about fanfic is you can enjoy sex that isn't technically possible or like science fiction, more so than porn videos.