(in this video, by mistake I call a stability ball a medicine ball, I am not a native speaker so sorry for that!)
having a good balance is not just about strength, it is also about mind-muscle connection, and when it comes to the core, most people use a lot of momentum because they are too stable in their position. one way to train the mind to take things slower is by adding more stability demand to the exercise which a stability ball can do massively.