Back in the day, we ran screen savers for more than just meditative entertainment, eye candy or news updates. We ran them for one good reason -- to save our freakin' screens! Burn-in was a serious problem on CRT monitors, and many displays ended up with a permanent menu bar shadow or outline of a high-contrast desktop pattern stuck there for posterity.
While LCD screens aren't nearly as likely to run into image persistence issues as older CRTs, they aren't immune to damage, as developer Daniel Sandler (author of screensaver Pyrothèque) learned to his dismay a year ago. After his flat-panel got munged by green static overnight, he whipped up a pattern-shifting screensaver to repair the burned-in pixels (based on Apple's recommendation of leaving an all-white image over the affected areas). Whaddaya know, it worked -- hence the $18 LCD Scrub screensaver/display repair tool.
Just updated to version 1.02 with registration and other bug fixes, LCD Scrub might just save you the cost of a new display. It's the fluoridated water of screensavers: secretly doing good behind the scenes.