As I am sure you know, Snapchat is a service that allows you to send photos that self-destruct after just a few seconds. The whole purpose is so that you can send embarassing content without worrying that it will come back to haunt you later. (A smaller use case is for students to send each other answers during a test so the teacher can't see it if you get caught - but I think/hope this is rare enough not to justify Snapchat's existence).
Other than self-destructing, it doesn't really do anything. So if it turns out that the photos don't really self-destruct, why would you buy and use it? The reason is pure motivated reasoning.
It turns out that there are a lot of ways around Snapchat. You can use an app like SnapHack or SnapSave that allows you to save the photo before it destructs. You can also search your raw storage and find it after the fact.
But users don't seem to care. Taking a rational long term view, yeah, they are not protected. But in the heat of the moment, you just can't help but send the selfie. Imagining that you are protected is enough to get over your decision threshold. Risk awareness is no match for motivated reasoning.