A rational company would, but in practice lots of companies either don't know how to measure productivity, or they are confused enough to believe that long hour

A rational company would, but in practice lots of companies either don't know how to measure productivity, or they are confused enough to believe that long hours will result in more output. So as an employee you want to ensure you have a hard limit on how many hours you're expected to work (40 for regular jobs, or as in this case, less than that).