What's the purpose of Mac firmware standalone updates still offered by Apple online? I'm learning Mac troubleshooting and have stumbled upon this article several times, which very clearly explains that you can download and install these updates from macOS itself:
About EFI and SMC firmware updates for Intel-based Mac computers (no longer updated since Published Date: 25 September 2017)

Just for the sake of knowledge, what's the purpose of these updates?  

These are older versions of the firmware, and you can't use them to update your firmware as you'll obviously have a later update, neither to downgrade the firmware. Neither to install the firmware if your Mac fails and upgrade (for that purpose there's an installation CD you can use).  

In what scenario would you use these?
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."