They're pretty obscure these days, but when I used to work for startups, I'd inevitably get roped into pre-sales trips, and, being before "the cloud", we had to lug all of our server kit with us. Much of our client base was pre-EU Central Europe, so a carnet was required. It turned every trip into a logistical flurry of heavy lifting, persuading airlines to take it as passenger luggage and obtaining endless rubber stamps from customs officials who were in no hurry to give you them (the UK was inevitably the worst at this...)