The actions you animate often follow an arc or slightly circular path. This is especially true of the human figure and animals. Arcs give animation a more natural action and better flow. Think of natural movements in the terms of a pendulum swinging. All arm movement, head turns and even eye movements are executed on an arcs.
This is the ‘path of action’ (or motion arc) and arrangement of inbetweens used in the above example. The ‘fairings’ used at either end of the path accelerate and decelerate the action which progressively slows down the movement towards the end – see ‘easing‘.