Because it's a lot of fun! It's obviously not everyone's cup of tea, but it truly is a great experience for those who enjoy it. It's not the same at all as jacking off by yourself. It's a completely different level of intimacy, and although "having an audience" is obviously not for you, some men enjoy the unique bonding time it provides.
I really enjoyed it when I was in my earlier to mid teens. It didn’t feel like it was about labelling anything, it was all about doing something all us guys liked to do, something we constantly talked about anyway, and instead of keeping it private, we just decided to do it out in the open among one another. I didn’t get to do it as much as I’d have liked since those friends moved away. During those moments I did have, we all never touched one another. It felt freeing to bond that way, we got to see what almost every guy is curious about: where does my size fit in among other guys? do we all jerk the same way or different? what do other dicks look like? (porn clearly isn’t the norm), mainly all questions most guys have which stem from the basic “am I normal or different, and what does it mean to be a guy?” I personally found my times jerking with other guys enlightening. I learned I’m very well endowed through that experience, I learned I cum way more volume than the average guy though that experience.
One of the things I really liked was that I got to learn that dicks and balls come in all shapes and sizes making every man unique: some dicks are long and thin, some long and thick (e.g. mine), some short and thin, some short and thick, some medium and thin, and some medium and thick. Some dicks have a long head (e.g. mine), some a short head, some a thick head (e.g. mine), and some a thin head. Some dicks have a big meaty slit, some have a narrower slit (e.g. mine). Some dicks are mostly straight (e.g. mine), some are rather curved, some dicks are thicker at different areas, some are the same thickness all throughout (e.g. mine). Some dicks have a foreskin (e.g. mine), some don’t, some dicks have lots of veins, some a few (e.g. mine), and some none at all. Some dicks are your same skin tone, some are lighter, and some are darker (e.g. mine), some dicks have pink heads, some have purple heads (e.g. mine), and some have tan/brown heads. Some dicks are big soft and stay big hard (e.g. mine), some are smaller soft but grow a lot when hard, some dicks get erect quickly like a spring-loaded machine, and some get erect slowly like a gradually stretching out tree branch (e.g. mine). Some testicles are huge egg sized balls (e.g. mine), and some are smaller grape sized balls, some testicles are oblong, and some are more circular (e.g. mine). Some testicles are in tight high-up sacks, and some are in loose low-hanging sacks (e.g. mine). Some guys cum a few drops, some cum a few hearty ropes, and some cum like a geyser (e.g. me), some guys ooze out their cum, some guys shoot it out (e.g. me), and some are a mix of those two, and there are tons of other things a guy’s genitals can be or have.
Mainly I learned where I fit in, that I wasn’t average at all in dick size, ball size, or cum volume and it felt good to learn about myself in comparison to other guys, since often we feel insecure about the size of our genitals or our sexual potential or we are just curious if we’re normal or are curious in general about others. And most importantly those times helped me discover my sexuality, but in no way did those times make me my sexuality, that is an innate thing. Those times just allowed me to learn about it by feeling free to be among other guys, because when we would jerk off in the same room together, there wasn’t any pressure or judgment, we were all just enjoying an activity we all did anyway but these times it just happened to be in the presence of others than alone. It’s all about the bonding experience, self-understanding, and self-discovery. I wouldn’t trade my social jerk off experiences as a teen for the world.