IMHO, their main advantage is that third parties can’t send email which appears to originate from my domain. I configure my domain to use SPF, so now spammers

IMHO, their main advantage is that third parties can’t send email which appears to originate from my domain. I configure my domain to use SPF, so now spammers can’t sign it properly. However, the fact that an email passes SPF verification only ensures that it was authorised by the domain owner. It doesn’t say anything about whether the domain owner is a spammer.