What is a purpose of a screensaver?

Back in the day, older CRT monitors had a problem with memory-effect -- if you left an image or text in the same position on the screen long enough, it would "burn into" the screen. Even if you turned the monitor off, you could still see the burned-in images.

To prevent this, screen savers change the display on a more or less random basis.

It's not as great a problem with LCD monitors, but a better solution exists -- use the power-saver option of the monitor to shut the screen down when it's not in use. Not only does this preserve the screen, it also saves energy and money.