The sync isn't really a backup as such i.e. you can't fully reconstruct your Joplin system using the data there but it does keep all your note data (so things like settings etc. are not retained). The sync is designed for just that, syncing notes between multiple clients so that they all remain the same.
Your Joplin profile folder is your working profile, it contains the database full of your note data, settings data etc.
If you are after a pure backup solution rather than a sync then have a look at the Simple Backup plugin - Backup Plugin.