I get the motivation in the abstract, but does someone have a use-case for where this is really useful?

I've worked on apps that are close to either extreme. I.e. static pages with content, and you add one or change one or two small pieces later, which is easily done returning the piece of html from the server, and data-exploration type apps where you initially load some kind of menu and then incrementally add/update/remove/reload all kinds of data, in which case your templating lives in the browser.