Screensavers became popular in the 90s because they helped combat a phenomenon know as screen burn. Screen burn is a permanent discoloration of areas on an electronic display such as a CRT display or computer monitor.
With the invention of LCD screens, the problem seemed to go away... but now it is back with a vengeance.
I think that due to the large size of modern screens... icons, folders etc are often displayed for long amounts of time, while a user works in a smaller window or browser.
Screensavers help by filling the screen with moving images or patterns when the computer is not in use. The longer you have your screensaver running, the more it will help with any screen burn issues you have.
Try our windows screensaver software or mac screensaver software for free to see how it helps.