Arc is an animation principle used to apply natural motion to objects. Mechanical actions, however, wouldn’t take from this principle as their actions tend to be in straighter motions.
The movement of a baseball player hitting a ball, however, would have to move in an arc – to get from preparing to hit the ball, the the bat rising high in the air as the player swings. Without arc in animation, this motion wouldn’t look fluid and instead look fake, mechanical, and not how it should – fluid and natural.
0:26 below shows short example of arc movement of the head