Because thousands of people already have an account tied to a specific email and are going to click the Apple button and get really mad when we can’t log them in.

So then you ask them for their email address and password once and link the accounts together?

And Apple Sign In helps this user, how?...

Apparently they want to use it, otherwise they wouldn’t, right? This way they can have the easy login using Face ID and you can use the account they already have.

Sending invoices, GDPR exports, validating that a user contacting you is a certain account, etc.

You send information to the apple address, that's what its for. You can still send it invoices or a magic link, the user gets it and clicks on it, nothing is changed in that regard. The difference is they can turn off that email address and never hear from you again if that is what they want.

Stuff sent to the fake email address will be forwarded to the user’s real email address, from what I understand. So you will still be able to communicate with them.