If you have to ask, then for you it probably wouldn't be fun. Not trying to be a smart ass here. It's no different than a guy who isn't into bdsm asking what's the fun in being tied up, or spanked or whatever. I'll never get anything out of a girl humiliating me, but some guys live for that stuff.
Basically, different guys get turned on by different things. Some guys like the dominant side of it. They love to see a relatively helpless girl getting plowed by one guy after another and they want to be a part of that. It may even play into a rape fantasy. Other guys like it from the submissive side. They love to see a girl being the center of attention, getting all the cock she wants, and they want to be a part of that. Other guys just want to get laid and it's cheaper than booking a girl privately. I'm sure there are dozens more reasons.
I know some guys think such a thing would be "gay." Other naked guys in close proximity and such. Of the few I've been to, there was nothing gay about it. If any of the guys were bi, there was no indication. All the sexual activity was concentrated on the girl.
Honestly, it's not really my thing. I really want to be into group play, and I periodically try again, but it just doesn't do anything for me. I tend to prefer 1 on 1 or 2 on 1.