By proving who sent the email (or, more precisely, which server did), Email Certification can hold the server owner responsible. If they send
spam, they get de-certified, which means in all likely hood, they lose the ability to email anyone at all. Spammers who can't get certified
can't send emails anyone will see.
I see you've never had your server compromised. And of course nobody can spoof an email header or perform a Joe Job.
These are just two obvious holes. There are certainly more.