My home-base is a private forum (DDL). I think of it as a place where people gather the best media from around the internet. We may not get it first but it eventually makes its way there. There are also many exclusive uploads (mostly encodes) but people do some fantastic work with upscales of movies and/or TV series. The user base is also part of the many private trackers the world has to offer. I am part of like 2 private trackers and they are phenomenal as well. I also use public trackers extensively as well as there is actually a lot of stuff out there. I will always try and seed for public trackers but I definitely don't go as long as I would for private ones (just cause I don't want to get dinged) (Yeah I could use a VPN but too lazy, even though I am in a place that don't give many fucks about media piracy). I prefer DDL because I don't have to maintain ratio, but I do understand that with private trackers there can be some more longevity (but isn't really guaranteed). There are also other private trackers of the index variety that aren't built around ratio which can be nice.
There can be a lot of rules with private trackers, and yes some of them are really annoying, not gonna lie. The rules are there for a reason, for the most part. For some it's about building a community, while for others it's about making sure stuff survives for as long as possible. There are so many tools for a fellow seafarer to have and private trackers are just one of them and they serve a purpose. I am of the belief that all information should be available to everyone forever, but the world don't work like that and the long dick of the law can really fuck you life up. I think a valuable lesson from private trackers is that it makes one not just be a leech and makes one give back. There is nothing wrong with leeching, but I always like to think that piracy is about sharing the wealth. (not trying to moralize piracy, to each their own)
But as you said, public trackers have A LOT of stuff. So much media can be found on them. Private trackers help if you want the best of the best with regards to certain media or for hard to find stuff. Each have their place and it all about using the tools available to you
A lot of the private DDL forums dont deal with any of that. It's more on the level of "If you know, you know"