It has to do with the CAN-SPAM act.
Here’s the relevant info
The CAN-SPAM Act, a law that sets the rules for commercial email, establishes requirements for commercial messages, gives recipients the right to have you stop emailing them, and spells out tough penalties for violations.
Despite its name, the CAN-SPAM Act doesn’t apply just to bulk email. It covers all commercial messages, which the law defines as “any electronic mail message the primary purpose of which is the commercial advertisement or promotion of a commercial product or service,” including email that promotes content on commercial websites. The law makes no exception for business-to-business email. That means all email – for example, a message to former customers announcing a new product line – must comply with the law.
The 4th requirement is the reason for the physical addresses in emails:
4. Tell recipients where you’re located. Your message must include your valid physical postal address. This can be your current street address, a post office box you’ve registered with the U.S. Postal Service, or a private mailbox you’ve registered with a commercial mail receiving agency established under Postal Service regulations.