The reason for this is that on private sites track your upload/download ratio. One way of doing this (and the most common) is to attach a personally identifiable code to the torrent file when you download it. Then basically the torrent you download says to the tracker "hey, it's me and I want to access this torrent". If the tracker doesn't see a personally identifiable code, then it assumes you're trying to rob the system and won't let you connect. So, you have to send them the .torrent file, then download it from them with a code on it that says that "it's you" when you access the .torrent file. Then the tracker lets you in and keeps track of all those good upload credits.