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);});