This performs routine wiki maintenance by removing old files that are no longer needed. Things like all of the old cache files that really begin bloating up the size of the entire wiki. Yes, it's good to keep some of that and not trim too vigorously, but not everything need be retained.
This is found on the Admin page under Additional plugins > File Cleanup. Press the Preview affected files button to see everything that will be removed, and then hit the Delete affected files button at the bottom if the list looks OK.
At the moment, there is no set schedule for this. It is not run automatically, and none of the other default settings have yet been adjusted. Generally, as with most big wiki operations, it is recommended—nay, demanded, begged, pleaded—to mirror the live wiki onto a locally-stored copy before using this plugin to delete stuff.