People are always interested in certain insider processes and want to see how their favorite products are made. People take yours to fabrics that make their favorite beer or chocolate and like to see how their favorite movies or music videos are shot. This is fun and gives you an opportunity to know more about favorite product or performer. The same thing with porn videos.
Sex videos are also interesting, both in terms of shooting and also in terms of knowing more about favorite actors and actresses. There are also a lot of funny or weird moments during the shooting, basically, as always. So no wonder adult content fans are always happy to see the backstage shooting, especially from the movies they already know and love.