What does the CategorySources plugin do? A little off-topic, but wondering if someone can shed some light on the CategorySources plugin.

A client site has hundreds of articles (in an 'articles' structure) that are currently assigned to categories via a categories field. It would be great if they could view the articles by category (which I assume is what the plugin is for?). I installed, activated, and selected the category groups in the plugin settings, but I'm not seeing any obvious changes to the CP. Where are these mysterious category sources? Or am I missing the point? The plugin docs are a bit light on specifics.
IMO, the plugin serves two purposes... one of which is simply to serve as an example for how to use the modifyEntrySources hook.

Beyond that, the plugin will provide you with an alternative way to view your Entries. You'll notice that the categories which you selected now appear as entry sources on the left sidebar under the Entries tab.

Oh, and Craft 2.3.2629 is required... that's when modifyEntrySources was introduced.