需要对app做以下配置，原理是什么，为什么需要这样做

var app=angular.module('app',[]);
//  lazy controller, directive and service 
app.config(['$controllerProvider', '$compileProvider', '$filterProvider', '$provide', function ($controllerProvider, $compileProvider, $filterProvider, $provide) {
    app.controller = $controllerProvider.register;
    app.directive = $compileProvider.directive;
    app.filter = $filterProvider.register;
    app.factory = $provide.factory;
    app.service = $provide.service;
    app.constant = $provide.constant;
    app.value = $provide.value;
}]);