One potential advantage is that many private trackers are meticulously-curated. The more people that are on a tracker, the harder it is to quality control every single upload. Most of the top-tier trackers aren't just a dumping ground for data, they have tons of categories and slots for each potential piece of data to go, and if a better piece of data can fit in that slot then the previous one needs to be reviewed, deleted, and replaced.

Another reason is that private trackers often have many rules to facilitate the overall health of the tracker and its swarms, e.g. minimum quality for uploads, minimum seed times, required ratios etc. If anyone could get an account they could break the rules over and over after being banned.