Obviously, swinging at the first pitch all the time is not a good thing. Hitters pick their spots to do it. The logical reason for doing it is pretty simple. The pitcher doesn't want to fall behind in the count, right? The easiest way to avoid that is to throw a fastball over the middle of the plate. If you're a hitter and you are expecting the pitcher to do that, is it smarter for you to take that down-the-middle fastball for a strike, or to swing at it?
It's all part of the game between pitchers and hitters. You can't just do the same thing all the time.