Which frameworks I should to use frontend or backend?

I have a question about JavaScript MVC frameworks (Angular, Backbone, Ember, etc.). Why I should use it for my app, because for example I have any kind of web-frameworks like RoR or any else which also use MVC structure. Is it possible to develop apps without any MVC back-end framework? Or I should use just in some special situations? May be now back-end developers have some extra tasks to do, and now I should do some work which they usually did. I'm really confusing.

I'm somewhat familiar with RoR and Ember/Angular. But still I do not see any difference between them.

P.S. I'm front-end developer, may be it will help you to explain me more correctly:)