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.