I am a contributor to date-fns, as I helped in implementing Indonesian locale and format ISO and RFC standards.
A web-based application can have a lot of pages. Each page can contain a lot of components and each component can have more than 1 state. In the real application, we might not always be able to simulate all of the states because of one reason or another, for example, lack of existing data or missing implementations.
One of the ways to bypass these missing links is to use Storybook—a tool that allows us to cherry-pick each component and have them rendered on a story page, where we can simulate all states of the said component in isolation.