javascript - When should $scope.apply() be used in angular?

What does $scope.apply do? In some cases, if $scope.apply() is not present, $scope.watch() is not triggered. For example,

The controller has the following:

setTimeout(function(){
  $scope.person = someperson;
}, 500);

person is being watched in the directive.

$scope.watch('person', function(){
  console.log($scope.person);
  $scope.apply();

});

In this case, watch is triggered only when apply is there.