But you're assuming there's some reason he must catch the ball. The requirement that the catcher stay in the catcher's box is important because it prevents the catcher from being an extra fielder in fair grounds. When no one is on base there are no disadvantages to moving the catcher to the outfield for instance, and letting every pitch bounce to the backstop. I guarantee you, if it didn't say it in the rules anywhere that the catcher must be in the catcher's box, it would be tried. So I contend it's important.