That's not the whole reason, though (at least for the original intent). Turning off CRT monitors means both a cycle added (which is not good for the electromagnets and capacitors) and diminished brightness due to cycling (this is due to the "warming up" needed to ensure that the screen is as bright as it can be, which takes up to around 30 minutes).
Turning off and showing a black image are not the same thing. Some old CRTs had a low power most where the power was on - thus the parts stayed warm - but no image was displayed.
Though too be fair, I think the low power mode wasn't signaled by a black image but instead turning off the sync signals or something like that. (I hope you get the idea that I don't know how it worked here)
In the early 1990s, there was no power saving. You could turn off the monitor with the front power switch, or use the screensaver.
A few years later, power saving options began to be introduced. The computer could signal to the monitor that it should go into a low(er) power state. Turning on again took at least 5 seconds for a visible picture, sometimes 15-20. That might not be appropriate if the computer had to be used in response to something like a phone call, or visitor at reception.
CRTS couldn't really show "black". It was more of a dull gray. That's what made the plasma display so amazing with it's true black. LEDs and now OLED are the same except they do not ahve the mass of a dying star.
The screen is charged. That charge causes burn in. The best way to not burn in, short of turning the screen off, is to charge the screen as little as possible. The least charge you can give it is a black signal, which means least burn in.
>The least charge you can give it is a black signal
The point is that you could not give it no signal without it being off. LEDs can do this. The closest to black from a CRT I saw was from Sony's Professional Studio reference monitors that were $32K for a 32" screen. When Sony brought out their OLED reference monitors, they did a side-by-side comparisson of their best CRT, an LCD and the new OLED. All 3 were receiving the same signal, and when the demo started black, the CRT was clearly "on" but the OLED looked "off" with the LCD in between. Just about the time I'm thinking to myself that the CRT brightness was turned up, they switched to reference bars and all were calibrated correctly.
Arguing that CRTs could display true black is arguing against history.
I think you've got the wrong end of the discussion, there.
If you can't or don't want to turn you CRT off... then what's the next best thing for avoiding burn-in?
Send it a black signal, or send it a bright signal?
It's the black signal, because it charges the screen less. Charging the screen is what causes burn in. If you charge it less you get less burn in.
> Arguing that CRTs could display true black
Nobody argued this. Where did you think you read this? I said 'show a black image'. Send it a black signal. It's the minimum signal you can send it without turning it off.
Or just send it a random signal, such as static or even better, an aesthetic screensaver. Showing black on a CRT didn't save any power, and the computer stayed on anyways (since it often took 3-5 minutes to boot up in those days). Black could mean something is wrong or signal is disconnected.