The concept and idea I derived from WordPress and it's way of creating plugins. Or what some refer to as event driven design. I personally prefer it to be used for making things extensible.
Each register requires a paramater that would pass is_callable(). So use your imagination :)
I've attached a simple example.php to further explain the concept. Also, take a peek at WP's documentation as it's pretty thorough. Keep in mind however that the api is not the same as pluginify's, it's just to explain the concepts.