1. Changing Locations. Pitchers need to be able to change the location of their pitches. They can't win an at-bat by throwing to the same spot every time. They need to be able to throw to different zones and eye levels. The more variety a pitcher has in their pitch location the harder it will be for a batter to assume where the ball will be thrown.
2. Changing Speeds- A good way to throw a batter off is by changing speeds. When you change your pitch speeds, the batter will be will baffled in their swing timing and especially if the pitcher is throwing out of the same arm slot (which helps with pitch tunneling and adds deception to the hitter). “A very simple pitching sequence that is effective is using fastballs to set up the off-speed pitch. This sequence normally involves getting two strikes with the fastball and then using the off-speed pitch to fool the batter for the final strike.”
3. Climbing the Ladder. A timely pitch up in the zone (not belt high) is great to sequence into an at-bat to help mess with the eye level of the hitter. The pitcher can be even more deceptive as they gradually increase their speed with each high pitch. Climbing the ladder with a four-seam fastball is effective on chase counts or as a setup pitch before you throw a changeup in the dirt.
4. Throwing Outside the Strike Zone. A pitcher with great control will be able to throw outside the strike zone and not risk getting it too close to the zone (to possibly give up a hit). Most youth pitchers are still learning how to keep it in the strike zone so throwing outside of the strike zone must be practiced intentionally. By successfully throwing outside the strike zone though, you cause the batter to chase bad pitches especially on counts where the pitcher has the advantage. This is a good pitch to throw when you need the last strike on an over aggressive hitter.
5. Experimentation, Scouting Hitters & Unpredictability. Learning how to pitch is hard enough on its own, but it becomes even more difficult when there's no experimenting (trying out pitches and seam orientations in the "lab") or study before taking the mound. The game of baseball is tough and pitching can be especially tough which is why you want to build your pitching repertoire in your team bullpens under the guidance of good coaching and instruction. Coaches and catchers (while a pitcher throws his bullpen) can watch for issues in the area of accuracy or velocity while the pitcher experiments with trying out a new pitch. Pitchers should always strive to keep developing. When they are doing research and trying new things, they should keep looking to improve themselves so they can have multiple pitches (by design) that they can throw effectively in games which will make them more unpredictable to the hitter.