Explain the need for DI in AngularJs?

Dependency Injection or DI, is a design pattern of injecting objects into the angular components instead of angular components creating for themselves. The advantage of this is, we can separate our web application in multiple logical units, we can reuse these multiple logical units anywhere in our application. In angular dependency injection is the main concept without which angular does not work. For example, In order to use services(built-in or custom or both) we have to inject these services in our controller as shown below.

angular.controller('myCntrl',function($scope,$http)
{
 //Use your $http service here 

});