The umpire's signals are what PA announcers look for when certain announcements
have to be made. We cannot legitimately announce a pinch hitter, pinch
runner, new pitcher or defensive changes until the home plate umpire
signals that the new player is in in the game.
Sometimes PA announcers get anxious and announce a pinch hitter before the umpire's
signal; when this happens the umpire becomes noticeably upset and will
often times gesture to the PA announcer his displeasure. Also,
in my opinion, some umpires' signals are far more understandable than others.
It makes the PA announcer's position much more tenable when the umpire clearly
points to the new player then points to the PA booth.