In converting each scene into an image, it’ll be easier to convey your vision of how the story should be told.
Storyboarding (like other stages in the video creation process) should be a time where edits are made and ideas debated. It’s extremely important to check and verify everything in each stage. It’s always better (and cheaper) to find mistakes or things to improve before going to the next stage.
In short, a storyboard will help you save time (and money) by making preliminary edits that would otherwise be made to the finished video.
For example, it’s much easier and faster to change the order of the story during the storyboard stage than once the video has been created.