For those of you out there not familiar with an animatic, it represents a “rough draft” of our cartoon. None of the art, music, or voice acting is final at this point, but what it does for us as creators is determine pacing, timing, shot order, and editing choices.
Animatics are used to make sure the final product will flow exactly as we want it to before we start work on the final art. Significant changes after final art has been created is more expensive, higher risk, and frankly harder to do. So the more accurate your animatic is, the better your set yourself up for success down the road.
Getting the temporary voices into the animatic help with pacing as well. Just because a line of dialog has a comma in it, a pause of a half second versus a whole second can change the entire implication of a moment.