Go to your favorite library. Take one book and destroy it. Is it still a library? Yes, of course. Destroy three more ... still a library. But what if you keep going? Clearly one book is not a library. So ... where is the crossing point?
The same is true of many jobs. Yes if I was vaporized tomorrow my team would still function pretty fine. But that doesn't mean my job is bullshit. It means that we're overstaffed for the average demand. It probably also means that 9-5 is a bit silly. But if you got rid of all of us you'd have a problem, and the company can afford a bit of slack capacity.