Have you ever seen some screens where there seems to be images or writing permanently fixed to them? This is called image burn-in and occurs when a monitor is left on for long periods without being turned off. In more specific cases, if the same image is constantly shown, over time it becomes burnt onto the screen. To prevent this from happening, you need to use a screen saver. Screen savers are images or graphics that move all over the screen, thus preventing burn-in as there is no set position for it on the screen.
Burn-in can be solved quite easily, of course depending on the severity. This can be accomplished by showing a white background on the screen for a protracted period of time. This is a time consuming remedy though which can be prevented by using a screen saver. So screen savers are very important. Never discount their usefulness. If you are not using one now, make sure you start doing so by the end of the day. Just as the name suggests, it will save your screen.