It's more akin to file diffing. Developers are notified of any differences in snapshots. The developer can they either accept the new snapshot, or investigate and fix.

Sort of - It ensures that your logic still generates the same output as before, so perhaps a step after the schema and a little closer to the user. I think people take it to mean the output will still look the same for the end user, though. That's rarely the case since markup is typically styled by CSS. CSS changes will largely be missed in the snapshots unless a class name is added, removed, or updated in the markup.