Running (aerobically) helps you recover faster, increases heat tolerance, helps remain alert, and increases the capacity to perfrom pitching. It won't necessarily help increase velocity, but it can help you work on the mound longer (during practice) which (if you're doing it correctly) will help you increase velocity.
To sum it up, the volume of training that can be tolerated in a session is improved; recovery from general training stress is enhanced; exercise within a training session is improved; recovery from general training stress is enhanced; sweating, and thus cooling is improved,; tolerance of the heat is enhanced; and the connective tissues associated with muscles and joints are strengthened.
One also doesn't want to negletc anerobic training, as exercises should at least be performed at the velocity of contraction of the sporting activity for any potential benefit to occur.
Meaning, training the body at the same speeds, movement pattern, contraction type/force of pitching off the mound will transfer over.