There must be some lines of work that frequently get refreshed with a new workforce for security reasons. I’m thinking something like road checkpoints in occupied territories, where the guards inspecting vehicles have to maintain high scrutiny behavior and remain alert. It’s probably been found that no matter how emphasized the importance of remaining on one’s toes is, the employee will gradually cut corners in their thoroughness and welcome laziness over time. This is largely due to the mundane and redundant nature of the work. A 1,000 or more cars get checked without incident, so it’s understandable that even the most patriotic of soldiers would lose their edge and slack off. Hence the necessity of rotating the crews and getting the checkpoints manned by people fresh from the lecture hall, still traumatized by images of death shown them by a shouting GI who almost breaks the slide projector glass when he slams them down for viewing.