Gene Deitch talks succinctly about why contrast is so essential to not only good animation, but all art in general.
An excerpt from Gene Deitch's book, How To Succeed In Animation (Don't Let A Little Thing Like Failure Stop You!).
The art of animation timing is related to a basic element of all art, and that is contrast. What makes a painting, a drawing, a sculpture, a building, a play, a movie, or a piece of music, or any object visually or aurally interesting and dynamic, is contrast: dark against light, large against small, blur against sharp, straight against curve, rounded against angular, close against distant, loud against soft, silence against sound, long against short, slow against fast, pause against action... Those last pairings are at the heart of animation timing.
When drawing the phases of animation the inbetweens you would only use perfectly even spacing if you were animating a machine. Nothing alive moves in evenly spaced increments of your 24-frame seconds.
In my book, I allude to this in general terms in Chapter 3, "Animation For Dummies." Here I will try to be more specific. There is nothing more important to animation than the precise spacing between your action phases your inbetweens. If you want to create impact, the spacing must be increasingly greater as you approach your end position of an action. If you want to create a "soft landing," your spacing must progressively decrease as you move toward your end position. In the complex series of moves within any scene of animation, the interplay of short against long, pauses against movement, close spacing against long spacing contrast of movement is exactly where you create the illusion of life comedy, drama, whatever.
Gene Deitch is one of the last surviving members of the original Hollywood UPA studio of 1946 and the instigator of the CBS-Terrytoon "renaissance" of 1956-1958. He was also: Animation Department Chief of the Detroit Jam Handy Organization, 1949-1951, Creative Chief of UPA-New York, 1951-1954, Director at John Hubley's Storyboard, Inc., New York, 1955, President of Gene Deitch Associates, Inc., New York, 1958-1960, Creative Director for Rembrandt Films, 1960-1968, and star director for Weston Woods Studios, Inc., Weston, Connecticut, 1968-1993. He has worked for over 40 years with the Prague animation studio, "Bratri v Triku."