I think of it as the "lawyer's light bulb", or the equivalent of "objects in this mirror may be closer than they appear" [which I actually thought was a joke when I saw it for the first time]
Leaving a light bulb on 24/7 will shorten its life, as it won't last as many years compared to if you just used it an hour a day.
This is, however, a 'fake' analogy, as it will still last the same number of hours, minus 'damage/wear' caused by the switching process.
Switching it on & off every second 24/7 will shorten its life still further, as the power-cycling will eventually cause early burn-out.
The analogy applies to both CRT & LCD monitors. The more you use it, the shorter its lifespan.
The lawyers would like you to be aware that you can contribute to the extension of that apparent span by using it less.