I've heard it gets used by rival airlines to identify you as a frequent flyer and therefore someone to be nice to, in the hope of poaching. So next time you are on a Skyteam or Star Alliance carrier, they can go that extra step.....

In reality, the only real use is that in a lost bag situation they've got some means of tracing you without having your address obvious. Have no idea whether the baggage people take any notice though, try to avoid my bag leaving my side......