OP here. Indeed. And importantly, this API makes it possible to restrict the query to a list of documents that the client doesn't already have from another sou

OP here. Indeed. And importantly, this API makes it possible to restrict the query to a list of documents that the client doesn't already have from another source. In practice, once you start using compound documents with an identity map, there are many cases where existing documents exist on the client and shouldn't be re-fetched. One really simple example is having a number documents that are related to "people" documents. For example, imagine a blog with blog posts and comments, each of which point at an author. When a list of comments for a post is downloaded, each comment will point to an author. Some of those authors will be the same, and some of them will already have been seen from previous posts. This structure allows the client to request only the authors that have not yet been seen (and if you're lucky, it may even be the empty set!).