Private trackers = rarer stuff and faster speeds.
you forgot to add that they are safer also

................
Private trackersA private tracker is a tracker which restricts who can use it, often by requiring registration of a user account. A common method for controlling registration among private trackers is an invite system, which is a system where trusted users are given the ability to grant a new user permission to register at the site. Typically invites, or the codes that are given to prospective users, are granted to users who have uploaded a pre-determined amount, meet specific ratio requirements, and have been registered. Private trackers usually register how much the users upload and download and may enforce a minimum upload-to-download ratio. As a result of the restricted access, their torrents usually offer better availability and speed compared to public trackers where leeching is more common.
Many private trackers are now implementing Passkeys in the torrent file, which gives each user a specific address to contact in order to get a list of peers. This helps to prevent unauthorized distribution of torrent files from private trackers.