That's a lot of heaving bosoms. There are probably a lot of reasons why people like to read about sex. First off, let's be honest, sex in fiction tends to be a lot better than sex in real life. Sadly, people often have enough of that in real life. This has to be one of my least favorite questions, because of course sex isn't necessary for fanfic.
Nothing is necessary for fanfic.
Fanfic itself isn't necessary. Neither are the shows fanfic is based on. But throwing in a sex scene isn't going to make a bad story better. The story should first and foremost be about the characters and their interactions, whether it is sexual or not. I have read some ghastly fanfics which seem like nothing more than Penthouse Letter rejects with names of characters added.