This is handy for accessing private methods but remember that things are normally private for a reason! Unit Testing is one (debatable) use case for this.
Example:
<?php
class Foo {
private function myPrivateMethod() {
return 7;
}
}
$method = new ReflectionMethod('Foo', 'myPrivateMethod');
$method->setAccessible(true);
echo $method->invoke(new Foo);
// echos "7"
?>
This works nicely with PHPUnit: http://php.net/manual/en/reflectionmethod.setaccessible.php