Paul wrote:

You can set the time constant on screen blanking, in the
Power control panel, and that will blank the screen after
20 minutes or whatever. If you move the mouse, it lights up
again.

Blanking a monitor won't save on power or lifespan of the LEDs or CFLs.
Blanking just means the light doesn't get through. The lights are still
one. Powering them off would save energy and perhaps lengthen lifespan.

You want to configure the monitor to power off after some idle time
threshold, not just show a blank screen. While a screen saver might
blank the screen, luckily the power options that you mean actually power
off the monitor. Well, not completely. The monitor is told to turn off
its display but the logic in the monitor is still powered so it can
receive further commands, like to turn the display back on.

There's also the Power button on the monitor itself rather than relying
on idle thresholds to tell the monitor to turn off only its display. Of
course, that means wear on the switch. However, I've found hosts where
the screen saver and power options for turning off the display become
ineffective. Despite the host being idle, the monitor stays on despite
vastly exceeding the threshold. Rebooting gets it straightened out
again ... for awhile.

I've also had programs that were running on a blanked out screen or when
the monitor's display was turned off that prevented me from seeing why
the host is refusing to acknowledge changes from the mouse or keyboard
to get the display back. I can see anything, the screen won't come back
up, and I'm screwed by having to do a power cycle (sometimes not even
the option to use the Power button for graceful shutdown will work).
While I might still have to power cycle to unhang Windows, I'd still
want to see what the screen might've shown me. Might provide a clue why
the hang happened. With a power switch on the monitor, I can just push
it and see the screen without relying on commands sent from a hung OS to
bring the display alive.