What is the benefit to joining private torrents?

A private tracker is a torrent website that provides the same functionality as a public tracker but is invite-only. This means you need to be a member to view the contents of the site and download its torrents. A tracker can either be semiprivate, where you can create an account for free by just registering your details, or fully private, where another user has to invite you. Within a tracker, there is usually an extensive set of rules covering how much one can download, what kind of content one can upload, what precautions one must take when logging into the site, etc. Such rules and content vary from tracker to tracker, and go from rather liberal with little enforcement to ultra-paranoid and autistic. Advantages of private trackers include:

• Speed: If you're familiar with torrents, you probably know that the bigger the swarm, the faster you download. Private trackers encourage their members to seed torrents for as long as possible, thus increasing the chances of a torrent having a healthy swarm for longer. Not only that, but many members use seedboxes, which are just servers based in data centers, offering very high speeds and excellent peering. Enough to max out anyone's home connection.
• Retention: Similarly, private trackers usually enforce rules that encourage long-term seeding. A few peers on private trackers will seed many torrents for obscure content that you wouldn't normally find any peers for on ThePirateBay or KAT.
• Selection: Some content simply isn't available on any public site and will only be found on private trackers. Sometimes you can't even legally buy it at all, ironically. Some trackers specialize in obscure or rare content, ensuring that it doesn't get lost from the Internet.
• Quality control: A major asset of private trackers, albeit one that can vary a lot across trackers. Good private trackers have stringent rules on the content format, quality, and organization. Music trackers will ensure you don't get horrible 92kbps transcodes; movie trackers will ensure you only get good encodes, ebook trackers will ensure you get retail quality, etc. Members and staff review and approve each torrent. Trumping rules and the removal of duplicates ensure you only get one, community-approved source for the specific content and format. This, coupled with a decent site layout, makes private trackers much more orderly than public ones.
• Security: There are two reasons private trackers are more secure, albeit they may not apply in the future. The first one is that most of them are obscure enough that no one really knows or cares about them (security through obscurity). The second one is that copyright trolls would rather focus on huge public sites that are easy to fish for peers rather than small communities that are hard to join. From a monetary point of view, it's more worthwhile to stop 10,000 casuals from downloading two torrents than to stop two neckbeards from downloading 10,000 torrents. There are some caveats to this though, copyright trolls will aggressively pursue the source of leaked pre-release media, such as screeners. As a result, many private trackers do not allow this content. Fortunately, leaked screeners are generally very popular and will be easily found on public trackers anyway. Large prolific piracy groups such as scene or well-known release groups are huge targets for copyright enforcement.

Continue to read this wiki if you want to learn more.

Some private trackers are shady. Even if you seed, they might undercount your contribution and ask for money. If that happens, it's a scam, just walk away.