Comment by thebrokencube

Quality control, and speed. This may not be as big of a problem as it used to be a few years ago, but private trackers give you quite a bit more assurance that what you're downloading is what is advertised. Also, since to be part of a private tracker you have to maintain the ratio, many users set up seedboxes and just leave their torrents on, while this doesn't seem to happen quite as much on public trackers.