My first PR to the DEV repository was when I added Storybook to the project. In fact, this was in March of 2018, when the repository was private (which means that I can't even link you to the PR where I added it!)
The TLDR is, Storybook allows you to build out components in isolation and test them visually based on the different states they could be in. Each story you write is the component in a different state.
I added some stories for some Preact components, but after that, Storybook never really got used. Fast forward to January 2020. I started working at DEV and I had some discussions with my awesome product designer Pawel, @pp, about the design system. I mentioned that Storybook was already in the project, but needed to be resuscitated. Once I got it back up and running, we started collaborating, building out design system components and some application components.