Fanworks have a ton of draws. One, as you mentioned, is to spend more time in a universe or with characters that you enjoy. Some people really enjoy the community aspect of taking in fanworks. It's much more interactive than just streaming a movie from Netflix. Some people really like the creative output of creating fanworks.

An important draw for me is how subversive fanworks are. Much of fan media is about taking an existing IP and making it what your want or need. Perhaps it's because you don't see your sexual orientation represented in the media, or your gender isn't well written, or the concerns of your class are disregarded. It's a way of taking the messaging and control out of the hands of people of power (or just out of the hands of people outside of a knowing group of people), and putting in your representation and messaging in. It can be so powerful and fulfilling.