screensaver is that moving images or patterns that appears on the computer monitor when it is left unused for a long time (or when it sleeps). The original purpose of screensavers was to prevent phosphor burn-in on CRT and plasma computer monitors.
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Screensaver is one kind of security measure,
For example, if you have to go to bathroom, but you also don't want to shut down your computer, you can just set the screensaver, so that when you leave and didn't move the mouse of touch keys it will show the screen that you need password to get in. So that if you got something important in your computer, you won't loose it because you went bathroom.