In a double play situation when the pitcher will start the double play through 2nd base...
Communication is critical. Before the 1st pitch, it is the shortstop's responsibility to tell the pitcher he will be turning two through him (the shortstop). Yes, the pitcher probably knows this. But baseball is about routines and consistency. You want to plant the seed in your pitcher's head so he knows exactly what to do with the ball when he gets it. Hesitation could lead to a busted play. When a ball is hit up the middle, both the shortstop and 2nd baseman are naturally moving towards the bag so it's important that the pitcher knows who his target will be.
This may seem like a picky detail, but coaches who stress small things like this will instill discipline in their players and probably avoid several errors over the course of a season.