You keep frontend UI changes that don't affect state in the frontend. If you don't need the server then you don't waste your user's time with trips to the server. If you need server, you can do things on the frontend at the same time you send the request, e.g. hide something, add/remove a class.

Sure, it's not for a smooth user experience over 2G connection, if that's your audience you'd use ordinary template rendering with Phoenix, or use it for a JSON API and build a JS client that talks to it.