Aroldis Chapman, with his ultra high-speed pitches provides a good example of why very fast balls are to a pitcher's advantage. First, look at his frequency of foul balls and swinging strikes on his off-speed pitches:
In this case, low speed is anything 96 MPH and lower. These are mostly Chapman's off-speed pitches. Note that very few of these pitches are in the strike zone. These swinging strikes tend to be pitches that fooled the batter into swinging at a ball. The following heat maps show his pitches 97 MPH and above:
These pitches are concentrated inside the strike zone. The swings and misses especially are located close to the center of the strike zone. These misses and fouls are great pitches to hit based on location, but batters just cannot catch up to them. If they take the pitch, it's a strike. If they swing, it's likely a strike. It's a huge advantage for Chapman, and a big reason opponents only hit .196 against him in 2010.