As an adult who uses snapchat let me try and explain to them the appeal.
Historically conversations were temporary. I would speak to you, and you would hear it. The ideas I expressed would stay with you, or not, but record of the conversation did not. It had a poetic quality no one considered because there was no other options. All conversations survived as memories and nothing more. Letters, crafted with pen and paper, invented a new form of communication that has real benefits to business and law as well as others. The thing is, letters took more work than talking. Until the 90's letters remained the primary form of documented conversation allowing memories to be attached - not to an idea - but to a real physical object.
Today we are smothered by our own words. People don't wait to see each other. Instead they share and send as soon as they think or see something. I don't mean to say this is intrinsically bad, or bad at all, but failure to wait decreases the content available for to face to face conversation.
Snapchat bridges these worlds. I can satisfy the urge to share right away without infusing an object with my thoughts. The conversation becomes more personal.