[eluser]Colin Williams[/eluser]
So, for a blogging system, the core functionality is create, retrieve, update and delete blog posts. This is all the core knows how to do, and all it is responsible for. However, it is built so that it broadcasts what it's doing, or invokes hooks in other ways.
Then, a third party module for comments can be written to implement these hooks and allow for blog commenting
without the core blog system needing to be updated.
Do you use Firefox? If so, I'm sure you've got some plugins installed. When you install a plugin, do you find yourself having to open up FF source code and adding calls to the plugin in the right places? I sure haven't!