Sometimes you are just angry. Angry enough that you decide to take it out on something or someone. Physically. This violent behavior may target a simple inanimate object, an opponent in a boxing match, or other similar situation, or maybe some convenient villain who happens to be in the area. Anyway, the point is that you are resolving your anger in a violent, but still somewhat socially acceptable way. See also Dartboard of Hate, in which the character takes out his frustrations on an enemy's image, and Punch a Wall, a subtrope that specifically deals with punching vertical surfaces.