What is Angularjs $compile function 2nd and 3rd parameter

I have this example

<div ng-controller="MyCtrl">
    <div compile-template ng-bind-html="post"></div>
</div>

And the angularjs code:

angular.module('myApp',[])

.controller('MyCtrl', function($scope, $sce,$timeout) {
    $scope.name = 'World';
    $scope.post = $sce.trustAsHtml("<h1>hello {{name}}</h1>");
    $timeout(function(){
     $scope.name = "3333";
    },3000);
});

angular.module("myApp").directive('compileTemplate', ["$compile", "$parse", function($compile, $parse) {
    return {
        restrict: 'A',
        link: function($scope, element, attr) {
            var parse = $parse(attr.ngBindHtml);
            function value() { return (parse($scope) || '').toString(); }

            $scope.$watch(value, function() {
                $compile(element, null,-9999)($scope); 
            });
        }
    }
}]);      

If you look carefully, you will notice this function.

$compile(element, null,-9999)($scope);

If i make it $compile(element)($scope), it does not work any longer.

Why is that?

Here is the fiddle.

http://jsfiddle.net/bugd67e3/4/