Private trackers have more torrents at better quality than public trackers. MAM has more books in more formats than anyone else. AvistaZ has more Kdramas than anyone else. This is the benefit of private trackers. If I want a book series, I know I can go to MAM and not only find the books individually, but also packaged together as well. I can get them in ebook or audiobook. I can have my choice of narrators, if multiple ones recorded the book.

You say you've never needed a private tracker, and that might be true. But what's also true is that you've made do with whatever scraps have been uploaded to public trackers, in whatever quality and format you can find. And you've never even had a taste of the choice that comes with a private tracker.