[zinekeller]

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).

[bluGill]

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)

[chrisseaton]

Show a black image, not turn off.