Consistency is extremely important in an animation.
In the case of a bouncing ball, if the size of the ball keeps changing randomly, the ball loses a sense of believability. Adjustments to scale can be a good thing - if they are deliberate. If the ball (or any object) gets smaller as it moves it will look like it is moving further away. If the ball (or any object) gets larger as it moves, it looks like it is moving closer. This is especially true if the position of the ball/object changes position closer or farther away from the horizon line.
There are many other aspects where consistency is important.
Characters rely on consistency. If the size or shape or position or sometimes even color of a facial feature or other body part keeps changing, the lack of consistency leads to a lack of believability.