I don't think "served from Rails" and "consumed by Ember.js" is quite right. It was designed to work with existing servers that have facilities for easily generating JSON (Rails, Django, various Node frameworks), and smart clients that want to index local data by ID (Angular, Backbone, Ember, etc).
In short, it's an attempt to extract the learnings about efficiently transporting a non-trivial number of objects over a REST-like transport in a sometimes-incremental way. Many different server/client combinations have been attempting to do this in an ad-hoc way for years, and Ember Data was simply an attempt to try something general-purpose out in the real world.