I never understood that. Apparently they use it as a primary way of messaging each other? At least that's what younger relatives have told me. I've tried to have them explain what makes the app designed to hide/delete stuff after it's been read better for communication, but so far haven't gotten an explanation I could make any sense of.
Yeah my brother says its the main way to talk to girls, which sexting was its original purpose so makes sense. But also the streak system and ease of picture messaging I think keeps people using it.
Nobody I know my age (early 30s) uses it any more. Instagram stories killed Snapchat stories. I don't know if Instagram has a disappearing messages option though.
I asked the same question, my friends all abandoned it for Instagram years ago, but I (an American) am seeing a lot of Europeans using it during my travels