The big picture is that angularjs is a framwork. A framework provides you different tools to build out your application depending on your needs. The framework can provide things like routing management for your pages, api request for your backend logic, IOC container for your dependency injection management, template rendering logic, and much more. Using a framework allow new developer who already understand the framework to easily work on large projects with lots of overhead. Developer use framework such as angularjs to build applications more quickly and efficiently.