So for some channels that provided no contact information, you now can acquire an email address, and for everyone else you may now get an additional one.
It also enables you to link multiple channels back to the same person.
Every bit of information you can get your hands on counts for social engineering attacks.
For very famous individuals this may also open them up to harassment. You can't find Elon Musk's private telephone number on the Tesla homepage for good reason. For that class of people, any time that sort of information leaks, they need to get a new private phone number/e-mail address.
I'm not sure that there are terribly many black market opportunities for "every bit of information" such that this should be a six figure payout or whatever.
- Regime critics with a channel on YT.
- Vulnerable individuals and others trying to keep their identity a secret. Putting yourself on YT means putting yourself in front of every deranged individual out there.
- Trump quite famously runs some of his own social media accounts personally, for better or for worse. And even where he doesn't, he probably retains ultimate control - in the case of YT it might be his personal google account that created the channel. He's probably not the only high value target to do so.
Also if you happen to be in any date leak, being able to figure out your private e-mail address gives attackers another place to check whether you re-used a password.
For any vuln you can make up a hypothetical one off usage. But to find the right buyer for that is effectively building a team ala The Great Muppet Caper.