Drupal 7 - When to use template suggestions?
page--internal-path is already defined by Drupal core as a theme hook suggestion, so adding your own version of it won't really have any effect (just as then removing that version won't have any effect).

You can see a full list of the core-implemented hook suggestions on the Drupal 7 Theme Hook Suggestions page. 

The simple answer to the question is that you need to provide your own template suggestions when you require that functionality, and Drupal's core system, or a contributed module, haven't already provided the overrides for you.