When you're working on an animation, even a short one, it's almost impossible to just dive in and get started animating right away. I've known a few people who could work right from a script and draw or model raw from that written description, but I've tried it before and let me tell you, the results were not pretty.

Using a storyboard will help you organize your animation, and match you mental visualizations of scenes with the written script; it can also give you a visual format to communicate your ideas to others.

A storyboard can be an elaborate, professional series of framed color artwork depicting action and motion in a scene, complete with written descriptions of dialogue, sound effects, and transitions into the next scene (these are most often used by studios for major projects)--or a single page of numbered thumbnail sketches, or even something as plain and simple as a quick series of motion-study sketches (as depicted here) to capture the movement of a body that you want to animate.

If you use a storyboard you'll find that you'll be able to plan your animations more cohesively with clear marker points to show progress, and you'll save yourself a lot of time and trouble when struggling to make the entire thing come together from beginning to end.