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)