Does it actually do anything more beneficial than the screen just blacking out/sleeping?
Screen savers are left over from the days of the CRT display. CRTs burn in, just like plasma, if an image is left on them for too long. Screen savers were designed to "save" the screen from burn-in by having an image constantly moving on the screen.
Now, since LCDs are in everything, the only thing that screen savers do is run down the backlight and shorten the life of the display.