signals the batter and nobody else. Any light or sound signal could be noticed by anyone else in the stadium (or the millions of people watching on TV, for that matter). Especially consider the catcher, who is crouching right behind the batter and is keenly aware of each pitch being called. And the umpire is right behind the catcher.

But a buzzer taped inside the batter's shirt will only signal the batter. And nobody else would have any idea - unless the batter's shirt gets ripped off during the post-game celebration....