why a screen saver is called screen saver?

sounds nerdy, right? well it might be it suddenly today this question clicked in my mind and searched the net and explanation was the one i expected. may be because this might have been in our graduation course material and i couldnt remember reading that.

well what does your mind say about this question? is it really saving the screen? seems dumb in the perspective of cpu usage. because the cpu can be in rest if your computer does not show screen saver. isnt it? ok. forgive me for stretching ....

"Before the proliferation of LCD screens, most computer screens depended on cathode ray tubes (CRTs). Images on a CRT monitor are generated using electron beams which are emitted from electron guns at the back of the tube, and manipulated by electromagnetic fields to form images line-by-line on the phosphorescent screen many times per second. In some situations the images displayed on the screen constantly change, but in other cases some areas of the screen, or the screen as a whole, change very little (the taskbar in Microsoft Windows, for example). When the same image is displayed on a CRT screen for long periods of time, the properties of the exposed areas of phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen. Televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent. Screensaver programs were originally designed to help avoid these effects by automatically changing the images on the screen when the computer was not in use (thus "saving" the screen)."

[explanation from - wikipedia]

[update] - congratulations to bangladesh for the fantastic win against south africa, the number one cricket team now. :D