As part of the animation class at UCLA's Animation Workshop, this excercise is taught as an example of the principle of Anticipation.
Anticipation is the concept that before any action, especially extreme action, there is an anticpation action that sets up the main motion. A very extreme version of this is the wind-up a baseball pitcher makes before throwing the ball. An example of a minor but still useful anticipation is the slight backward motion one's arm makes before reaching for something.
If animated actions do not have anticipation at all, they can look mechanical. If the thing that is moving is supposed to be more alive than mechanical, then anticipation should be used. Anticipation can also be exaggerated for humorous effect. A slight anticpation helps the viewer "see" the action more clearly because it tips off the brain that something is about to happen, even if the viewer doesn't remember seeing it except as part of the overall motion.