It goes back to the days of touring bands heading overseas with a truckload of equipment, especially before the Common Market (as we used to call it).
To avoid being charged duty going in/out of various countries, you'd start with a Waybill of some sort, a docket listing (in detail) all the kit you were transporting.
You'd be checked on arrival in each country, and as long as you still had it all when you checked out then nobody would hit you for Import Duty, whether in Spain, or the States, or coming back home.
A decent 4-piece band might be carrying, say, six guitars and three basses, plus a ridiculous amount of other gear. The chance that anyone would bother to check serial numbers was pretty remote.
Now compare that with a tourist arriving at Heathrow with an expensive hardcase for a bass, strolling nonchalantly through the Green Channel and whistling ...