Delivery time is the total time the ball is in the player’s hand from initial movement to the release of the ball. It can be broken down into segments, namely set to reach back time and reach back to release time.
Reducing delivery time helps to reduce time to plate and makes it harder for runners to steal bases. In addition, it is important for a pitcher to keep his delivery the same across different pitch types. This makes it harder for batters to pick up on different pitches being thrown.
For position players, reducing time to deliver the ball to the base reduces the distance a baserunner travels while the fielder has the ball in his hands.
Real Life Example:
If a pitcher has a different movement between say, his fastball and curveball deliveries, it may be detectable by the batter so that he will know what pitch is coming.