1. The pitcher can see the runner(s) better
2. The motion is faster so the runner, who might steel, has less reaction time
3. The pitcher can throw to any base without stepping off of the rubber (with some restrictions)
(The following steps are for right-handed pitchers. Everything is the exact opposite for a left-hander.)
(STEP 1) The pitcher places his right foot parallel and touching the rubber and his left foot parallel about two feet away. His hands are separated.
(STEP 2) The pitcher places his left foot beside his right. At the same time he brings his hands together to a set position.
(STEP 3) The pitcher raises his left knee to a 90°.
(STEP 4) The pitcher pushes off of the rubber and separates his hands into a throwing position. He plants his foot approximately one yard away from the rubber directly toward home plate. shortly after his left foot is planted, the ball is released.
(STEP 5) After releasing the ball, the momentum of the follow-through takes him one more step.