QR codes in Rooms are used as invites. For example, if you start a room that's invite-only, you share the QR code with people you want to join it, which gives them access. Unfortunately, QR codes are necessary for open rooms, too -- regardless of what you join, you need to either a) scan the QR code from an outside source, or b) take a screenshot of the QR code within the app, then move through a series of steps for the app to recognize it and allow you in.
I'm not sure why Facebook chose to use QR codes over a string of numbers or letters, but it's a hassle. I imagine many people will give up before they even get to join a room and see what the fuss is about.