WordPress has quite a few hidden features. My favourite so far: MU Plugins, or “must-use plugins”. Introduced with WP 3.0, the code says:
* Check the mu-plugins directory and retrieve all mu-plugin files with any plugin data.
* WordPress only includes mu-plugin files in the base mu-plugins directory (wp-content/mu-plugins).
* @since 3.0.0
* @return array Key is the mu-plugin file path and the value is an array of the mu-plugin data.
Modularity requires additional overhead. That’s just the way it is. If you want to semantically separate different components of your web app or theme, you have to include files, run safety checks (e.g., include_once), extend components that you only use 20% of, etc.
WordPress encourages this behaviour, if you have plugins that only effect your admin panel, the files still get included and the actions and filters still get included, the functions are still defined, for every frontend visitor page render — but they are only called into action when you’re on the backend of your site.