Snapchat is useful and popular because it is a way for people to say hi to their friends without having anything to say. You just take a selfie with a filter, make a face, and you're done. This lets you feel close to your friends without having the pressure of actually having a conversation topic.
The genius of Snapchat is that because photos are ephemeral, the bar to create content is as low as it gets, which results in orders of magnitudes of more content creation and sharing compared to platforms like Instagram. On Instagram, you spend all of your time obsessing over the perfectly angled and filtered photo, hoping to get as many likes as possible. As a result, you'll post on Instagram once a week, but you'll post 5 times a day to snapchat. That's a two-orders magnitude of difference in usage, which is why Snapchat is so successful.
He's handy with a camera, so lots of content, introspective comments, tripod pics of his profile while staring out at the sea or what not...
Every new follower brings him a couple more.
I don't know anyone else who uses it, but the rest of us are boring married people who don't go to bars much (and not for the same reasons if we do). A couple of the marketing guys at work send stuff out on it for the company, I gather, which is really weird to me since my impression is that it's (still) mostly used for sending clips of drunken stupidity and nude pics or hookup arrangements. They definitely don't use it to message grandma—that's what Facebook is for.