A lot of people use a screensaver on a daily basis for something other than the intended purpose. Screensavers have been around since the early 1980’s which goes to show that they must have been made for an important reason. Some people look at screensaver’s as a way to conserve power or look at it purely for its entertainment value, but there really is a practical use for them.
CRT monitors (you know, the big and heavy monitors) use electron beams to light up phosphors in order to display an image on your screen. The refreshing of the screen happens several times per second, and this could cause problems if the computer isn’t used for long portions of time.
If the monitor is constantly being refreshed with the same image, this can cause the phosphors to get "burned-in" and never fully return to its normal state. It kind of reminds me of when your parents told you that if you make that silly face for too long, your face is going to get stuck that way. Same thing goes for the CRT monitors, but I’m sure you’re wondering when this would happen?
There are probably several portions of your screen that don’t change very often, such as the Taskbar, which could end up getting burned into the screen. This effect is called "ghosting." Another time it could happen is when your computer sits at a login screen (especially at offices and schools where computers can go untouched for long portions of time). When I was searching around for an image of such an effect there was one that came up for an airport terminal, and it is among the worst that I’ve seen:
This is where a screensaver comes into play…it constantly refreshes the screen to ensure that the phosphors are not being "burned" with the same image. LCD monitors and some of the more modern CRT’s are exempt from this problem because of how they are created, but it just goes to show that there is a real reason behind using a screensaver.
Looks like this truly is a "screen saver"!