DMARC is essentially an opt-in to strict mode. Primarily it prevents other people from forging email to look like it is coming from you. The goal of dmarc is to prevent other people from impersonating you.

> 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.

The biggest benefit I saw in a small domain was greatly reduced backscatter spam. Before someone’d randomly make up a billion emails on my domain and send “from” them, and I’d get various out of office replies, etc (catch all) - that basically never happens anymore.

Because if someone spoofs an email coming from your domain DMARC tells the receiver what to do with the spoofed email.