So far, animation frame only allows to jump from one frame to another in a brutal way : frame 1, then frame 2.
There are many situation in which it could be useful to make frame 2 appear after frame one gradually, like a cross over. To,achieve this effect, I usually use a video editing tool to,create the crossover and generate a succession of image. Then, a cross over effect uses about 3 to 5 frames depending the smoothness I need.
Imagine a option that allows animation to play with a cross fade effect between to frames, It will allow to dramatically reduce the number of frame needed and optimise memory use. It may also allow us to make better animations.