He explained that in large plants such as a telephone exchange where there were hundreds of racks of essentially identical equipment that the only important indicator when all the equipment was working was a lit green bezel at the base of the rack immediately above the plinth to indicate that the rack had power. Thus a tech peering down a row of racks all of whose bezels were alight was informed immediately that all those racks were working. If a bezel was not lit then power to that rack had failed.
Indicators on individual items of equipment in the racks remained off unless a fault developed within the unit upon which a red indicator would light up. Maintenance and faultfinding was much quicker if the presence of a light indicated a fault rather than no light at all. If the tech peered down a row of racks and saw a red light he knew exactly where the fault was located.
This went hand in hand with what's known as the fuse indicate which is a led or globe in series with a resistor and wired across a fuse. Only when the fuse failed would the fault light appear (because of its simplicity such indicators are highly reliable).
Simply, using too many indicators is confusing. Unfortunately, these days, the baubles, bangles and beads brigade of designers who use leds for just about everything has never understood this. Similarly that green indicators should be used to indicate power is present and red to indicate fault. Unfortunately, no one bothers with these conventions anymore.
The Christmas tree panels in submarines and in just-in-time manufacturing plants are similarly designed—strategically located/placed lights on the panels only light up to indicate trouble (such as water breaching a watertight compartment, etc.).
I've never understood why such an important subject as human ergonomics isn't mandatory in university engineering degrees.