Private trackers are the communal libraries where enthusiasts work hard to improve the collective archives. They sort out the people who cannot behave and the whole experience is a lot better: rules regarding organization and filenames etc are being followed, tags are added so that you can search efficiently, requests are filled, quality is ensured, holes are plugged, people don't upload duplicates, you'll find help and exchange in forums about the tracker's topic etc etc, all because it's a private club which includes the possibility that your membership ends if you pee into the kitchen.