@larryi best to ask Audiogon that but my assumption is this site exists primarily to provide users the service of buying and selling things. You have to have to provide a valid address for that in order to avoid fraudulent or incorrect addresses that would cause issues completing transactions smoothly. The forum is just an adjunct service. If this were just a forum and no financial transactions involving shipping addresses were involved there would be no need to provide a valid address. Furthermore the forums could be used to facilitate address fraud which is a real thing. Just look it up. You can’t join Audiogon just for the forum as far as I know. So you must provide an address and the address needs to be verified.
Hope that helps.