I agree with your assessment of Snapchat. We're too old to understand it. Best I can do this conversation with my niece (13 y/o).
me: why do you use Snapchat?
niece: everyone is on it. Why aren't you using it?
me: can't you do the same thing with Facebook Messenger or any of the other chatting programs? I mean some of these "cool" features have been since ICQ and AIM.
niece: I-see what? You're a dinosaur like my mom, you still have a Facebook account.
me: you don't have a Facebook account? But you have an Instagram account that's own by Facebook
niece: yeah, but I only use the Instagram to read the stupid things people do like this "guacamole lady". I never post anything on Instagram.
me: so you use Snapchat because it's not Facebook?
niece: kinda, but mostly because the stupid things I say and do probably won't come back to haunt me when I use Snapchat. Messages disappear. And aren't you the one telling me to scrub my online present and watch for my privacy.
me: (teary eye..she actually listened)
niece: I have a school fundraiser
me: damn it, here's a $20 get some chocolate for yourself
TL:DR version: people use Snapchat because messages disappear, network effect and it's not Facebook.