> 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.

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account42 3 hours ago | parent [-]

[citation needed]

	▲	cube00 an hour ago \| parent [-]
		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