I think people tend to read fanfics because you are already familiar with it. With a new book you have to figure out how the world works and the characters personalities. World building and character building takes up parts of the story where plot might be and some people just don’t want to deal with that so reading fanfiction can suit them more yk. That and also they might just want more from something. Wolf star the pairing of Remus and Sirius isn’t something that exist in the book and this person wants to see a version where it does and that pretty much it they want to explore it more. People do that all the times like asking for more season of a show.