The MAC address is a globally unique identifier of the network card in the computer - hence it's pretty useful to uniquely identify the machine/user. It's also very stable; unlike cookies or data stored in the registry it won't be changed by the user or removed by crapware removal programs.
We've developed a browser extension that checks for better prices, user and professional reviews as well as videos on e-commerce sites. It only becomes active once we find something useful. Our primary market is Germany currently but we're looking into other countries since we have reached 85k downloads in AMO already.