(At least) two reasons. First, once you already made your own personal copy, and you already have your torrenting strategy in place, adding another torrent is b

(At least) two reasons. First, once you already made your own personal copy, and you already have your torrenting strategy in place, adding another torrent is basically as easy as just dropping it into a folder. Second, exchanging content. You upload shows X, Y and Z since you have them and hopefully you can get access to A, B, and C from someone else who happens to have them. Though, this is more prominent in private trackers and risker in public ones.