It's just another form of KYC:

1. If it involves a cell phone number, it's usually tied to a sim card which is tied to your "real" ID.

2. Email is normally extremely compromised and can usually be traced back to you.

Therefore, it's pretty friggin easy to for any nosey party to come after you for some dumb notion of "tax evasion."

Plus, it's only another means of adding more attack vectors to possibly compromise your security.

In short, it's dumb AND dangerous.