There is also quite a bit of reliance on the check-in process for Z/employee fares, etc. It's typical that FAs/pilots/etc will commute on other airlines, and often will "spray" fares around depending on flight loads. IE, you need a connecting flight to your home base, the one you booked is oversold by two seats and almost everyone has checked in, so you book another flight that leaves an hour later, but show up at the airport for the first flight in case there's a seat available, etc.

Ultimately your employer only gets billed for the flight you actually board, so there's low risk in spraying out and checking in for several flights. Also when flying standby, prioritization for many airlines involves check-in order (and also they'll often prioritize paying customers, friends/family, then their own employees over other carriers' employees, or similar).