Everyone of a certain age has a favorite screensaver, from that one where the blobs bounce against the screen getting closer and closer to hitting the corner, to that one with slightly different blobs doing exactly the same thing.
They are still used today, but unlike old blobby and the squiggly line that wriggles around, they no longer serve a purpose.
So what was the original purpose of screensavers? As the name implies, it was to save your screens.
In older cathode ray tube (CRT) computer monitors and televisions, if you left the same image on for too long it could become burned into the screen, leaving a permanent ghost image of what it was displaying. On TV, where channels would stop broadcasting overnight and put up a display card instead, if you left your set on, you could wake up to that card ghosted in your screen forevermore.
Continuously displaying the same image caused the phosphor coating on the inside of the screen to burn in and stop normal function, making it unable to be re-excited to display a new image. The solution was to display a screensaver, which kicked in after a certain amount of time and constantly changed what was displayed.
Though we no longer use screensavers for that purpose, people continue to use them for their other benefits, such as hiding what you have left on the screen from nosy colleagues or family members.