It's fun.
In my case, it doesn't replace civillian sex or relationships. Rather, it's just another avenue to explore sexuality. And it really is another avenue, as it's quite different from what you get in civillian sexual relationships. There's a certain thrill in picking a girl from a roster, seeing her, and then moving on with your life (or better yet repeat if the chemistry and potential is there, while keeping things no strings attached). You can try different fantasies, different body types, etc. The girls are pros, the agencies are pros and don't exploit the girls, and the rates are fair.
It's a win-win deal in my eyes, so whenever I'm single I dabble in it.