Ephemerality is liberating. A large portion of social media use is not about exchanging information (which would be useful to persist) but about socializing. Just as you probably wouldn’t feel comfortable if every conversation you had with your friends while hanging out were recorded, a lot of users (particularly young users) feel more comfortable expressing themselves when they know with reasonable certainty that their communications are not being recorded online. Its often for sharing moments and making jokes and hanging out, not for conveying actionable information.