I think the “history” aspect (of preserving old RPMs) is relevant; there’s also an aspect of incompatible content, where some repos publish drpms (for example) and some don’t. Strange pulp3 error - #19 by mhjacks is one of the threads that preceded that change in Katello to deal with the different potential challenges there.
So if I understand right, “Content Only” by default gets RPMs, comps, and bootable tree stuff but ignores some of the potentially problematic content (which would also affect metadata); “Complete Mirroring” pulls everything in the repo including metadata.