BMTP and SMTP use different server ports, so a system can accept just SMTP, just BMTP, both, or neither. This lets recipients register their preferences with their own system manager, not a global opt-out list somewhere else.
Authentication allows the recipient to identify the sender.
The separation between BMTP and SMTP lets the manager of each server decide to accept non-bulk mail via SMTP, bulk mail via BMTP, neither, or both. Authentication makes it possible to implement schemes where the sender pays the recipient to accept the bulk mail. Every other form of advertising requires that the advertiser pay for access to potential customers, so this brings bulk e-mail into line with other advertising media. I suspect that a lot of users would be willing to accept a lot of junk e-mail if the senders paid enough that their ISP account was free. BMTP is designed to be easy to implement.