QR codes are just handy automated ways to pass addresses around without errors. Easy to scan with some random device to another random device even if they're not connected.

I don't know how popular they are but they're popular enough that these scam addresses have (or had) decent money in them (?). I wasn't really looking to use one of these sites, I just wanted to know the proper QR code data format so I could generate my own. That's when I noticed most of the sites were giving me the wrong address. I imagine like anything, people use them because it's convenient for example if you want to put a QR code on a website or something and your client can't do it (like me, I don't use the GUI wallets). If you were using the QR as a donation address you might not even realize it's not working.

It's relatively easy to identify the fake ones. Go to the site and enter an address then scan the resulting code with something like qtqr or SecScanQR that shows you the raw data. See if the address matches. If it doesn't then you can look that address up to see where all the coins are going.