Context for those who haven't worked in the field: A PLC is a programmable logic controller. They are typically programmed with ladder logic which grew out of discrete relay based control systems.
Generally they're controlling industrial equipment of some sort, and making changes without a thorough understanding of what's happening now and how your change will affect the equipment and process is frowned upon.
I interned briefly at a company which mainly built industrial control systems. One of its most interesting features (which is also very mind-bending if you're coming from any sort of typical programming ecosystem) is that every "rung" is evaluated in parallel. (As a physical relay-based control system would have back in the day.)
I remember reading a story like this from the early days of Acorn. The first production sample of the BBC Micro came in, and would crash unexpectedly. Trial and error found that connecting a jumper wire between 2 particular points on the board stopped it crashing, but nobody could work out why it crashed or how that fixed it. They never worked it out and ended up shipping mass quantities of the BBC Micro with the magic jumper wire in place on each one.
Generally they're controlling industrial equipment of some sort, and making changes without a thorough understanding of what's happening now and how your change will affect the equipment and process is frowned upon.
https://en.wikipedia.org/wiki/Ladder_logic