HTML is great for declaring static documents, but it falters when we try to use it for declaring dynamic views in web-applications. AngularJS lets you extend HTML vocabulary for your application. The resulting environment is extraordinarily expressive, readable, and quick to develop.
So you're working on a design and you need a mobile screen mockup or maybe you want to post one of your designs to Dribbble and you need that sexy screen surround to get those likes.