What is Storybook used for?

Storybook is used for developing, testing, and documenting UI components in isolation. It allows for some sort of visual interface where developers can create and demonstrate pieces of an application individually from the remainder of the application. This empowers builders to make, test, and iterate easier on individual components since every component can be viewed, modified, and tested in all types of states and with different types of inputs.
Storybook proves very useful in design systems and component libraries by allowing teams to create a consistent, reusable set of UI components. It also acts as an interactive documentation that developers and designers use in collaboration with ensuring behavior and styling for components remain consistent across applications.