Edit: So they took a service that was as simple and flexible as could be, put arbitrary restrictions on it, and wrapped it in pretty UI. Kids are dumb man. (I'm 26)
In a world where everything you do is recorded and stored long term the promise of something fleeting is a beautiful promise.
MMS doesn't even come close to that experience.
Snap Stories are also very common. I don't want to tell someone explicitly what I am doing, but it is still fun to share slices of life, so I will record a few seconds of an event and post it to my snap story. If someone wants to know they can. Usually about 30 people watch each of my stories.
1) With my girlfriend we just say hi and see a little bit of each other's daily work lives in each picture
2) With my buddies on inside, fleeting jokes that require the context of a picture and some art work