When to use Final in PHP

Question

Although I am aware of what a final class is, I am curious about how and when it is actually necessary.

<?php
final class Foo extends Bar
{
   public function()
   {
     echo 'John Doe';
   }
}

If I've got it right, "final" allows it to extend "Foo.".

Can somebody tell me when and why to use the word "final"? I.e., is there any justification for not extending a class?

It would be useful to construct a class that extends "Bar" if, for instance, classes "Foo" and "Bar" are missing some features.