Comment by lucideer

This is true of many many unions, but it is neither a necessary part of a union, nor exclusive to unions.

At my company, the majority (all?) employees in my local office are un-unionised. We have the same system (it's just departmental). This is a system that emerges in large old bureaucracies regardless.

Which is why it's present in many unions tracing their formation back to the early 20th century. They're big and they're old.