I was just reading through the interview process for RED, and they specifically forbid the use of VPN during the interview. I don’t understand this requirement, and it seems like it would just leak your IP address to the IRC host, which could potentially be used against you in a honeypot scenario. Once they have your IP, they could link that with the credentials used with the tracker while you are torrenting, regardless of if you used VPN while torrenting.
For the known top trackers, it’s to have more control over nefarious users such as duped accounts, banned users and potential law enforcement trying to sign up. Sure it’s not a bulletproof solution to deal with bad actors but it’s still an effective way. You gotta remember that these trackers are free (yet private). The admins/operators don’t get paid, so last thing they want to do is waste time dealing with such users.
At the end of the day you still have to trust them with your real IP, but it’s mostly to protect themselves afaik. I obviously can’t speak for every tracker out there.