It prevents somebody from finding a boarding pass, or being handed one from an accomplice, or using one that was just lifted off somebody in the bathroom from getting into the secure area. Anybody who flies knows if you want a boarding pass there are lots of people who either lose them, or whatnot, and anybody with a printer and a third of a brain can print up an acceptable e-ticket receipt to get past at the airports you don't need a boarding pass from (or the places that let you print your own at home).