Screen Savers were a ubiquitous part of technology long, long ago, but are no longer a mandatory component to computer ownership. It's possible that, as an Old Person, you haven't realized that you don't need it anymore.
Screen savers were necessary as they prevented screen burn (or "burn-in"). The prolonged display of non-moving images would ruin the monitor, sometimes permanently. This is not a problem with today's technology, so today's screen savers are used for entertainment or information—for example, it might provide time, date, and temperature.
Personally, keeping a screensaver on your PC or laptop seems like a waste of battery life or electricity.