[cube00]

> What would be the benefit of setting up DMARC on top?

Some mail providers will junk your mail if you don't have a reject/quarantine DMARC policy because you're seen as enabling the spammers so everything out of your domain must be punished.

[account42]

[citation needed]

[cube00]

When properly configured, particularly at an enforcement policy of p=quarantine or p=reject, DMARC can positively impact inbox placement.

https://mxtoolbox.com/dmarc/details/email-deliverability