Ask HN: What happens when people start saving Snapchat pictures?

I am just trying to understand what this Snapchat is all about. It seems like the catch is you exchange pictures of yourself that is expected to last no more than 10 seconds or less. Why the demand for this? I think for a demographic where they want to exchange photos but don't want it floating around on the internet forever or in other people's phones. What if the photo lasts forever or lot of them begin to adopt persistence by means of other apps? Would they flock to a new platform?