Well, it is a parameter of the Scraper base class ( https://github.com/mozilla/mozdownload/blob/master/mozdownload/scraper.py#l81 ), which in turn gets called by the DirectScraper ( https://github.com/mozilla/mozdownload/blob/master/mozdownload/scraper.py#l491 ), so I'd think so. 

It throws a TypeError when I leave it out...Why? Is it important?