"What's the point of being boyfriend and girlfriend in middle school?"
There's social value in such a thing even for the young. It's a shorthand and widely understood way to say that they're wooing each other when "it's complicated" is still far away in terms of people around them understanding. It's also practice for more mature relationships and there's the "cool" factor in having a steady when you're that age.
There's also a value in being "taken" - especially when your peers are still figuring out things like flirting, cheating, and what kinds of conversations are appropriate with friends. Being "taken" means you can see your friends go through all those awkward "ask your friend to ask her friend to ask her (totally not seriously, but seriously) whether she wants to go to the dance with you" and the associated drama. It gives you an easy way to deflect attention from would-be suitors who haven't quite figured out how to flirt and so they monopolize your time at your locker and obsessively analyze your every word. It also means that when the hormones do start raging, you at least know what direction they're supposed to be raging in - dating
a girl "proves" you're straight. (It does nothing of the sort, of course - plenty of gay
men and lesbian
women had girlfriends/boyfriends, respectively - but it's a social cue and your peers will usually respect it). And when you are interested in trying something like holding hands or kissing, you have someone to try it with whom you know won't shoot you down for asking.
Most of all, it's proof that you're date-able - having a boyfriend/girlfriend can be an important status symbol, and your social reputation usually gets tied to theirs. Date someone popular and you get more popular, etc.