I think you may be unfamiliar with Snapchat because Snapchat has some of the richest of interactions of any messaging or social network. You're literally communicating to your friends with videos and photos. The only thing that could be better than that is being in person. For some users, they're sending Snaps to dozens of friends every day, that they wouldn't otherwise talk to.

It makes texting/email/fb feel so boring and almost formal. I feel comfortable Snapping my friend I haven't talked to in a couple months something random, whereas I would be less likely to hit them up randomly on text.

Stories, the Snapchat broadcast medium, also let you feel more connected with your friends and loose connections. I don't need to talk to some friends every day, but it feels nice that I can share these silly, mundane experiences with them.

This is a much richer experience than browsing my friends' highly curated photos on their Instagram, their boring/activist/humblebrag FB statuses, or their pure text tweets.