Ute I’ve had my account with Revolut ever since, but now they want my tax ID number, so I’m wondering what that’s actually for—I don’t have any business dealings with the tax office!
That's a common desire against money laundering and so on. If I remember correctly, while registering to Revolut you were asked to give your German tax ID or to give it later, when they ask again.
Since Revolut provides a German IBAN, they want to have a lot more additional stuff from you, if you want one, that's why I keep my Lithuanian IBAN as long as possible.