angularjs unit test when to use $rootScope.$new()

I don't really understand when I should use the $rootScope = $rootScope.$new() in my angular unit tests. This snipped you see in many unit test examples. But in the following example that doesn't work:

angular.module("app.root", []).factory("rootFct", function ($rootScope) {
    $rootScope.amount = 12;
    return {
        getAmount: function () {
           return ($rootScope.amount + 1);
        }
    }
});

The associated unit Test doesn't work:

describe('Amount Tests', function() {
    var rootFct, $rootScope;

    beforeEach(function() {
        angular.mock.module("app.root");
        angular.mock.inject(function (_rootFct_, _$rootScope_) {
            $rootScope = _$rootScope_.$new();
            rootFct = _rootFct_;
        });
    });

    it('Set Amount in rootScope on 10', function() {
        var result = rootFct.getAmount();
        expect(result).toBe(13);
        $rootScope.amount = 15;
        result = rootFct.getAmount();
        expect(result).toBe(16);
    });
});

it only works when I am changing the

$rootScope = _$rootScope_.$new();

to

$rootScope = _$rootScope_;

and so I don't really understand when to use the $new() and for what its good for?