What does the CategorySources plugin do?
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.