The main benefits of private trackers are:

• Download speed. Many users of these trackers will use seedboxes to build ratio. This generally results in a faster download speed for peers.

• Security. Many (but not all) private trackers have strict entry barriers, such as invite only or application based signups. This keeps copyright trolls out of their swarms, which eliminates the need for a vpn or other method of masking your identity. Depending on where you live this can range from a nicety to a necessity.

• Longevity. Torrents generally live longer on private trackers.

• Community. Some private trackers have a forum or IRC channel where you can interact with other community members.

• Availability. Many private trackers will have a wider range of releases of a single media.

• Quality. You will generally find higer quality releases on private trackers. That's not to say that high quality releases don't make it to public trackers, some do and some don't.

• Faster releases. Releases will typically come to private trackers first. May torrents originate on these trackers or come from scene groups and trickle down.

If you're finding everything you want on public trackers then you probably aren't missing anything. You could test the waters on TL or something next time they open.