These programs would display the same thing on the screen in many areas. This would cause the screen to be nearly readable even when turned off. Screen savers were used to help prevent this type of burn in. Today computers are used much more for different programs, and the screens are less likely to burn in.
Screen savers are now a novelty, a distraction from a boreing screen.
This would not be so bad in its self, but screen saver operation tends to exercise the computer a lot more, and seems to cause more window errors. If you are running a screen saver, and get windows type errors, try removing the screen saver for a couple days and see if the errors don't occur less often, or even stop.