为什么要加个call，个人觉得多此一举

Question

forEach = function( ary, fn ){

  for ( var i = 0, l = ary.length; i < l; i++ ){

    var c = ary[ i ];

    if ( fn.call( c, i, c ) === false ){

      return false;

    }

   }

}

forEach([1,2,3],function(i,el){

    console.log(i,el);

});