I can understand using tor on random websites, since you can’t tell which is tracking you.
With the fediverse you don’t need any PII, only email which you can get by pretty easily. you probably give up your ip, which doesn’t mean much when it comes down to it. However I’m no web expert, so tell me what I should know.
If you self-host yourself, you don’t give your IP address or personal private data to anybody but yourself. Data that gets federated to other instances is public by design, like username, avatar and posts. Maybe the list of followers etc.
Tor is useful if the instance does not know who you are. Once you register to a website, you lose half of Tor’s benefits since the site already has an idea who you are.
I’d like to think tor is just a precaution, like signing up for an email as a whistleblower, you’d like to have your location more hidden.