Good exercise. Preexisting tropes, goals, clichés, and lots of room for creativity. There's a sort of framework already in place, and you just sort of work within it. Like writing based on a set of instructions for an exam. Much more gratifying than struggling to write original content and getting nowhere for A) lack of practice and B) being unable to keep up with the evolution of said content in my head over the years due to lack of practice. With erotica you can just grab this trope and that cliché, add one or two special elements and suddenly you're producing stuff.
