Why, indeed. The short answer–because we need one–might sound a bit smart-assed, but it’s true. Nobody would spend time debunking things like spirit mediums, quack cures and creationist nonsense if they didn’t waste money, harm people and generally retard the progress of the human race.
But if you want a longer, more interesting, in-depth response–complete with fascinating historical examples–read Daniel Loxton’s two-chapter essay “Why Is There a Skeptical Movement?” Loxton is a Canadian skeptic and editor of Junior Skeptic (the kid-friendly section of Skeptic magazine.
I’m really hoping this is a sneak preview to an upcoming book-length work.