What's the purpose of Mac firmware standalone updates still offered by Apple online?
Generally speaking, these firmware updates are for reliability, but as EFI/UEFI become more and more complicated compared to BIOS, there are also security vulnerabilities that require patching.

One good example is the fairly recent Thunderstrike 2 rootkit vulnerability.

Apple has not included optical media for system restoration since Mac OS X 10.7 "Lion."