I've been using torrents for over 5 years. At first I used public because I didn't know there was private. I got 50 kB/s ~ 200 kB/s down speed from public so I'd get a 700
MB movie down anywhere from 3 hours to a over a day. Lots of people on public
trackers are greedy. They cap their upload speed in order to download as fast as possible. And when they're done they hit & run. When I downloaded my first private torrent I got close to 1
mB/s ~ 1.5
mB/s and took me about 10 minutes to get a movie. I was blown away with the speeds and
seeders stayed on the torrent after they got it. My IRL friend (he lives a 30 minute drive away) got two letters from his
ISP about downloading copyrighted material (from public
trackers). I've had none, zero, zilch.
I could list a whole plethora of reasons, but personally those two are my main reasons.