It changes the dynamics of the guessing game to "can I hit this pitcher on this pitch" which can easily and just as quickly turn into, "I'm 1M in debt to a bookie and if I screw up this game enough where the bookie makes his spread, I'm out of my debt." - aka throwing baseball games, just like the Black Sox scandal a century ago.
It gives the hitter a distinct advantage at the plate, knowing what is coming - and when to swing and when to hold off.
Thank you - no one’s ever explained it that way.
But what about the third base coach flapping his arms and legs around... Who is he signalling and how does it help