Quote from the page: “Decoupled web projects involve so many components. The hard part is testing all the pieces locally, together. Netlify Dev delivers this b

Quote from the page: “Decoupled web projects involve so many components. The hard part is testing all the pieces locally, together. Netlify Dev delivers this beautifully with one command.” What are these "decouples web projects" and "components" and "pieces" that are hard to test locally, together?