If you're building macOS apps, it's common to want to test them on all system versions you support. Especially so considering Apple's attitude towards backwards compatibility.
Virtualizing an older ARM version of macOS was never going to be sufficient to QA x86 applications running on older Intel Macs. For that, you'll always want real x86 hardware.
EOL or no, there are many people who still use older OSes because they own older computers, can't afford an upgrade or don't want one (it works just fine!), and can't be bothered with figuring out running a newer OS on officially unsupported hardware.
Case in point: I built a macOS app that implements Google's Nearby/Quick Share (an AirDrop-style file sharing thing on Android). Multiple people tried running it on Catalina and were disappointed that it wanted a newer OS. So I did end up backporting it to Catalina.
CI is the big one, or similar testing against older versions for backwards compatibility. Usually good enough to just compile for it on MacOS, but sometimes we get a surprise in a third party library or something that only shows up on an older release.
I've first tried recompiling the Keychain app, but it had too many dependencies that were not trivial to build, so, using an older Mac, in that case, was the easiest way to get the private key from my own keychain.
You don't need a Hackintosh to virtualize a Mac - you can actually download the MacOS image directly from Apple and boot it right into QEMU with the proper configuration. I've used a few scripts over the years that could have an OSX image running on Linux in less than 15 minutes.
13.4 was released on May 18, 2023. That's actually not very far into the past.
Anyway, what would be the most common use cases for this? And how common are those?