It's a more intimate version of social media that is typically only used by smartphone-savvy people and it's perfect for this. I see nothing besides my friend's content and a few ads between viewing their stories, while instagram inserts ads and algorithmically sorts content in the feed. Once I go through every one of my friend's stories, there's nothing to keep me on the app and endlessly scrolling like instagram. To me and others, insta is intentionally addictive Posting permanent posts there about your life can sometimes come across as vain, while snapchat is more of a window to view your friend's world through and gives very open statistics like who has seen a story, if someone has opened a snap, etc.
I have my problems with snapchat, like poor android support, slow startup time, bloat, and sponsored stories on the right side are tabloid trash. But it's still very heavily used in America. Most friends outside of the US say that it declined in use in their European or african countries like a fad, but it's become a standard in the US. Those of you saying it's useless besides teenagers using it to sext or not knowing anyone above 18 that uses snapchat look out of touch or like you're pushing an agenda. It might be declining but it is still a large force and very used!