So you've got a machine at another location that you need to be able to operate but, due to Mutphy's Law, you never enabled Remote Desktop (RD). And, again due to Murphy's Law, the local operator can't enable RD. What do you do?
Enable Remote Desktop remotely using the registry editor and Connect Network Registry option. This allows you to enable Remote Desktop. Then, reboot the machine and you're able to connect.
Here's a good writeup on how to do this: http://oreilly.com/windows/archive/server-hacks-remote-desktop.html.
hat makes them so useful is that you can pack a lot more information into a small area. Combine that with the ability for most smartphones to scan and decode these codes and you now have a way to pass information along very easily. You can generate a QR code for just about anything, a web site (URL), contact information, a calendar event, etc.