You'd be amazed at the effort that goes into curation and comprehensiveness on a good private tracker. Because ratios are so important there is a strong incentive to upload obscure stuff that is missing. Additionally, the ratio economy can be tweaked for things like providing subtitles (including translations where there have been none before). There's a private film tracker that I'm aware of where I believe half of the members must be film studies professors. They will post monthly themes on obscure genres such as "queer cinema prior to 1950" or "Italien cinema under Mussolini" and these will literally include hundreds of films.

It's so impressive that I think some of these private trackers could reasonably argue in court that they are engaging in fair use for academic study. It's archival work at a scale that would be unimaginable if restricted to recognized institutions.

private trackers usually offer much faster release time than public trackers (shows are available less than a minute after pre on the good trackers, which often is about a few minutes after airing), have tons of good seeders so you can get pretty much all files very fast (even if its a bit old), and content that is otherwise not available elsewhere at all (for content-specific trackers mostly, such as music ones)

also, release groups usually dont have their own trackers, their releases are generally ftp'd to a few topsites and distributed from there (some private trackers get directly from the source thus having a slightly faster pre time and are more desirable/exclusive, others get it further down the chain of distribution)

The unpopular old stuff will still be seeded when you want it a couple of years later, since (almost) everyone that has downloaded it and hasn't thrown it away want's their ratio's up. They won't stop seeding it until they remove the files.

So the main reason to do it is high download speeds on old unpopular torrents.

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.

Private trackers have much more carefully curated content than public trackers. It makes for a much smoother download experience. Not to mention that the downloads happen as fast as your connection can support.

I haven't used public torrent sites in years - the private option is so much more attractive.

They are often faster (higher ratio of seeders to leechers), and are much less likely to have swarms of bots connecting to their torrents recording all the connected IPs so they can send threatening letters to ISPs/users.

Private trackers have per-user torrent keys and are often even locked to your IP address, so anyone who is not authorized (MPAA) will not be able to see any peers (a list of who to send scary letters to).

Curation and organization is one. For example, search a good movie tracker for Pulp Fiction and you'll get a dedicated page for that film--including stuff like poster image, YouTube trailers, and IMDB link--with torrents organized by quality, resolution, special features, and so forth.

The other is, well, you're wrong about the long-tail stuff. TPB, for example, has lots of audiobooks but is very short on printed material. A dedicated book tracker may have a much better selection of fiction, textbooks, and other sorts of specialty books (tabletop RPGs, for example).

Private trackers get scene releases hours (maybe 1 or 2) before they show up on TPB. On good trackers the number of seedboxes can max out any leecher's connection. The best trackers race to supply the content the quickest. This is called tracing or torrent racing. The groups with public trackers are p2p groups. The scene does not use torrents. http://en.wikipedia.org/wiki/Warez_scene Watch tracing live: https://trace.corrupt-net.org/live.php

Curation and organization. Continued seeding of really old stuff.

This is like saying why do you need niche news websites like HN, when anything mainstreem will be on the 3-letter channel and everything else will be obscure, unpopular stuff.

It's true, private trackers generally are "obscure" if you consider every person in the world (just like the majority of the world doesn't read HN). But if you niche down to your basic interests, then there are good private torrent trackers that will generally have almost everything worth having in that area.