The default camera app in the last two major Android updates and iOS have had the ability to scan QR codes for at least the last two years. This is what we used for our referral codes at makerfaire a couple of years ago.
The MagicMirror post I did a while back also uses a QR code for the guest wifi, but since this is a screen and not static- I can rotate the SSID and the password from time to time. But as @Tom_A said. This is a convenience feature.