URLs are opaque, and can often serve as very useful IDs, but, alone, they imply a one-at-a-time model of fetching documents, and this spec is trying to provide

URLs are opaque, and can often serve as very useful IDs, but, alone, they imply a one-at-a-time model of fetching documents, and this spec is trying to provide a way to easily request only the documents a client needs in a compound document. Keep in mind that this spec actually requires that every ID be able to be readily converted into a URL based on information found in the same payload, so URLs are still front-and-center in the design. It just separates out the notion of a unique identifier, so that it can be used in other kinds of requests.