Tweening is a technique that I noticed for the first time in products like Flash and ImageReady many years ago. It allowed an animator to set a start and end state of an object and determine the number of frames in which the transition between the two states should happen. Individual states were then automatically calculated and not manually created through object duplication or other inefficient means, which often led to a smaller file size. So finding ways to convert at least part of the animation in tweens could have been considered as an optimization.