"Except for those few line drives just before the doughnut hole, more exit velocity is generally better for batters. Although exit velocity most directly affects slugging percentage — harder-hit balls go farther and turn into extra-base hits more often — it is also correlated with almost every other positive indicator of offensive performance. It even affects how pitchers approach a given hitter; it’s positively associated with walk rate, presumably because pitchers fear throwing balls in the zone to high-exit-velocity hitters.
There are many things we can’t know from these two metrics alone, of course. Anyone who’s ever seen Billy Hamilton leg out an infield single knows that speed plays a significant role in what happens after a ball is struck. In fact, we can see the importance of swift base running by looking at ground balls and grouping batters according to their speed score, a Bill James-designed composite of stolen bases, triples and runs scored. The higher the speed score, the faster the runner."