Graeber's got a blind spot about this.
The first ENG sound gig I ever did (where all your gear is strapped to your chest), I found myself mixing for the jib at the end of the day. The jib is MOS. This means the jib gets no sound. Yet there I was.
Because the sound supervisor had judged me to be incompetent.
He wasn't wrong. I had no idea what I was doing, no one had walked me through anything, and it was basically a training day for me. And it's not like there wasn't going to be sound for the jib if I wasn't there; if I wasn't the most incompetent sound mixer on set, it would have fallen to someone else. Generally it's considered an honor - you get paid, even though you're not really working. Does the jib maybe need sound? I mean, it could happen... and supervisors get itchy when there's crew hanging out by craft service for hours at a time.
The security guard who guards an empty room? I'll bet the museum's insurance company has a guards/square feet rule and I'll bet his boss doesn't rotate duties because a couple of his employees suck at their jobs and it's far easier to make a couple idiots guard empty rooms than it is firing them for cause and dealing with union grievances.
The guy at the front desk of the dorm in the summer? I'll bet there's a very expensive mothballing procedure that gets really inconvenient if they need to do maintenance and it's so much easier to get a work study kid to sit there. It's literally a "you're a warm body in case someone needs to be here" job, explained that way, because it's waaaaaay cheaper to have a caretaker than to close up for the summer. This is all explained in the opening to The Shining by the way.
The temp surfing Lynx? There's a receptionist on maternity leave that wanted everyone else in the department to (A) recognize how important her job is (B) how she's the only one who's good at it so that when she comes back they'll be happy to see her, not deciding she needs to do a whole bunch more shit to justify her paycheck.