Why do you have to have a return address on the top of a business letter?

The first reason you put a return address at the top of a business letter is good manners; the same as when you meet someone you tell them your name or if you're making a business phone call, you tell them who is calling.

The practical reason that you put a return address at the top of a business letter is so that a response can be sent. Even if the sender is not asking for or expecting a response, the recipient may have a need to reach you about something you may not be aware of related to the subject of your letter, or even to thank you.

If you send a letter to a business without a return address, your letter will not be given the appropriate attention or credibility if the recipient believes that you are trying to be deceptive or remain anonymous.