In recent years, live action filmmakers have started making pre-visualizations before shooting their films, because it’s such a great way to see your shots before actually going out and shooting them. This could save a lot of time since you know if the scene is going to work ahead of shooting.
For animation it’s a necessary step. Animating takes so much time, that you wouldn’t want to waste days and days working on a shot only to find out later that the sequence doesn’t work. After storyboarding and making an anima tic, previz is the last step before diving into full animation.