You suspect one of your cylinders is missing. (not physically, just not firing)
You pull and reseat all the wires one at a time. You get a worse idle on every wire pulled, except for one of them. That is the cylinder that is giving you issues.
Because, on the other plug wires you pull, your engine runs more roughly, or just dies. Obviously, that cylinder was pulling it’s load, and when you pull the wire, it is no longer working. But on one of the wires you pull, there is no discernable difference. That is your problem child.
Could be anything from a broken cam lobe not allowing gas-air intake, to an actual issue with the wire or the plug, or any other number of things. But, at least you have it narrowed it down to one cylinder.
Of course, back when I worked on my own car, you were expected to change the plugs every year. Seriously, when is the last time you changed your cars plugs? I don’t even think that todays engines use their spark plugs. I think they are magical!
And if you are driving anything that is 40+ years old (like my wife), this is a perfectly good troubleshooting method…