If you have to ask yourself why out of the teems of top-notch freeware image editors at your disposal, folks consistently choose IrfanView, then you don't know IrfanView.
It may not be the slickest-looking app by a long shot, or even the most complex. But its combination of simplicity and strength make the classic image viewer and editor a middleweight favorite. It doesn't hurt that the feature set has some unique extras in addition to the regular touch-up tools. Check out IrfanView in this First Look video.