CRTs can suffer a thing called "burn in" where if a static image is left for a long period it can permanently burn that image onto the screen. Screen savers were devised so that there is always a moving image being drawn to the display to circumvent the possibility of burn in. Modern displays aren't susceptible to burn in so screen savers have mostly fallen out of use. Plus energy saving considerations just put everything to sleep on fairly short intervals.
I have a friend who does not like when his phone screen goes to sleep during a call. He probably turned sleep off because every single phone he has ever owned has had the call screen burned in.
Just take a look at any old arcade game with a CRT when it's powered off. You can still see the title screen burned in. I worked in an arcade for a few years repairing and setting up game cabinets. Our workshop where we kept spare parts had some leftover CRTs from decommissioned cabinets and I always knew what game they came from.
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u/clearhit 7h ago
Wait that's why they are called screen savers
...I always just thought it was for aesthetics