Seeing how the UI would look and interact with it without having to spin up a starter template and a stable internet connection eventually became a necessity so I could, even when I'm travelling, create UIs and see how it would actually feel when the app is finally built. This also meant creating mocks and MVPs offline became viable.
The playground uses both Polaris Web Components and Polaris React, with active work being done to ensure we can retire Polaris React as soon as possbile. Polaris Web Components are in Beta and a lot of components are still missing, and this repo will evolve over time before a full migration hits the production repos.