Why would you ever want to do that? Well, if you've adopted Apple's new scrolling paradigm in OS X Lion - "the so-called "natural scrolling" you may have noticed that the setting does not affect scrolling with the keyboard. If you've gotten used to moving window content up and down with touch gestures, you might want to maintain that metaphor when you scroll using the keyboard. Install NatKeyScroll, and the Up and Page Up keys move Web pages up, while the Down and Page Down keys move them down. Makes sense, doesn't it?
The extension has a few settings, which can be found in Safari's extension manager.