The fastball at the 7 or 8 will prevent the hitter from keeping their front-side in on off-speed pitches, making it more difficult to track off speed pitches down and away. Purpose Pitches will also open up a bigger percentage of the outer Go Zone on ensuing fastballs.
When you command fastballs in the Purpose Zones, you will be more likely to get away with a mediocre curveball in the middle of the plate because the hitter is seeing two pitches coming out of the same slot during the first half of the balls flight. The same slot refers to the numbers 7 or 8 that a bigger curveball starts to break during the ball’s flight before ending in the Go Zone. When you follow a Purpose Pitch with a pitch away from the hitter, it is much harder to hit.
If you can't execute fastballs up in the Purpose Zones, the hitter can recognize the curveball earlier because of the height of the pitch. If hitters aren't seeing fastballs in the 7 or 8 Zones, they can eliminate any other pitch in that slot and recognize the pitch earlier. Therefore, fastballs at the numbers 7 and 8 will make all pitches away harder to hit because the hitter has to respect the fact that you will throw a fastball inside off the plate.
When a left-handed pitcher throws a fastball hard and inside to a right-handed batter, it becomes much harder for the hitter to adjust to the change-up on the outer-half of the Go Zone and vice versa with a righty pitching to a lefty. What is the opposite of a hard inside fastball? It’s an off-speed pitch away. Changing speeds and locations will disrupt the hitter’s timing more often than not.