Think of it this way. When you swing streamers through a steelhead or salmon run, it's very important to cover water. You cast, mend, let the fly sweep, and then step downstream. You don't cover water if you stay in one place. If you move too slowly, you telegraph to the fish that your fly is coming, letting them know something's wrong. If you blow right through the run, you might miss fish, but it's actually better to move quicker in this case. Ideally, you want your fly to just "appear" and the fish to suddenly see it and decide to eat it. If the fish sees something slowly sweeping back and forth in the near distance, chugging its way downstream like it's on a pendulum, that's not going to work.