I use them to signal in pitches and to give signs from 3rd base coaches box. Offensively I use them because the way I do it, I have 9 different hit and runs, 9 steals, 9 squeezes, 9 signs for each 1st/3rd double steal play, etc. Sure, you could guess right in a given situation, but you won't steal my signs. I'll change the sheet in the wrist coach every week or so and always have a different one when we play someone twice.
As for the catcher's, I have a grid with 60 fastball signs, 40 curveball, 40 change ups. Once again, our signs are not going to get stolen then relayed to the hitter. We do not say the numbers out loud, we signal them in. Had a coach say them out loud against us a few years back in the state semi-finals. My hitter figured them out and hit a 3-2 curveball out to win the game.