If your class uses the pimpl idiom, you can avoid changing the header file on the public class.
This allows you to add/remove methods to the pimpl class, without modifying the external class's header file. You can also add/remove #includes to the pimpl too.
When you change the external class's header file, you have to recompile everything that #includes it (and if any of those are header files, you have to recompile everything that #includes them, and so on)
