And using these methods, vs purely PHP, allow for the application to do things faster and offer a nice, snappy experience, and probably loads of other benefits that perhaps Koop, Nacin, or someone smart could elaborate on. I just know I like using the new tools in WordPress built this way. I also know that there are a lot of outdated plugins that haven't updated from instantiating thickbox media modals to the new media modal. And that is in part because of the lack of developers' experience with the tools.
It's time for me to stop, because I'm already beyond my pay grade. But if I want to stay in the game in a few years, I imagine I'll need to change that.
So, the purpose of this post is to show you some resources for learning. Here's a sample plugin by Thomas Griffin (Soliloquy slider author) on integrating the new media manager in a plugin. And here's another post by Mike Jolley (lead developer of WooCommerce) showing how to use the new media manager in a plugin.
More generically, check out this post about these frameworks getting into WordPress core in 3.5. Also watch this video tutorial by Jeffrey Way on understanding Backbone and the server. And best of all peek into this Code School class on Backbone if you're ready to get started on learning Backbone.