These days it's all about modularizing and separating concerns.
Angular let's you create a front-end completely decoupled from a backend and backend agnostic.
Angular is also in fact completely agnostic about your model. You can plug in any kind of model as long as you can parse it in javascript.
With this kind of architecture, it's a lot easier to do things like re-use a backend for multiple front ends, for example a mobile-site and a desktop site and a native iOS app and a blackberry app. It's also easier to maintain and change each individual piece.
JS frameworks are only a front-end though, you also need to adapt your backend into a modern API style, rather than a html spitter-outter.