In 1687, Professor Isaac Newton wrote Principia, the most important Physics book of all time. While the majority of Professor Newton's work exceeds the physics required to understand pitching, his three laws of motion explain how pitchers should apply force to their pitches. When Sir Isaac Newton determined that moving objects continue moving with constant velocities in straight lines, he refuted Aristotle's theory that objects were in their natural states only when they were at rest. Now, moving objects are also in their natural state when they move with constant velocities in straight lines.
Baseballs start 'at rest' in pitchers' gloves.
However, gravity is an external force that constantly acts on baseballs. To hold baseballs at rest, pitchers counter gravity's force. Therefore, while pitchers apply force to baseballs to achieve maximum release velocities, they continuously overcome the downward force of gravity. Once baseballs start moving out of pitchers' gloves, they want to move with constant velocities in straight lines. This wastes force.