Why there is a screen saver in all systems?

They helps save your display from burning in an image.

There are very few monitors on the market any longer that will actually suffer from image burning, something that DEFINITELY occurred with old CRT monitors.

Screen savers now are mostly just a toy. Occasionally they can be used to insure privacy and perhaps a little security, but monitor protection is no longer an issue.

JB, you probably rembember to old ATM's, you could read the Menue with the Display powered off.
Not a Lot of People use Plasma's for Monitors? if they do, being phosphor based, they should avoid static screens. They are improving all the time.