DMARC doesn't introduce a new security mechanism by itself, but it fixed a few bad security flaws in SPF and DKIM. Without DMARC, SPF and DKIM do not add meaningful security.
DMARC updated and formalized how receivers should handle authorization and authentication of inbound email by adding the alignment requirement. Alignment ties the SPF and/or DKIM domain to the rfc5322.From domain. This also allows receivers to use domain reputation rather than IP-reputation.
SPF by itself has a bad design flaw that completely breaks any security that SPF was supposed to provide. By adding DMARC this security hole is fixed. (Although SPF is still horribly unreliable, it should be considered legacy at this point, don't rely on SPF).
DKIM benefits from DMARC alignment requirement too. DMARC ties the rfc5322.From domain to the DKIM signing domain, which makes DKIM practical for sender authorization, not just authentication.