"Or for macOS/iOS application developers to send compile jobs to."
That's basically what I bought my Mac Mini for. Needed to make sure my cross platform application worked on Mac OS as well as Linux and Windows, and something to build it with. Didn't want to get a Mac with a screen, mouse, and keyboard, I already had a KVM setup with spare ports. Didn't have much spare space on the desktop for aa big machine. The Mac Mini was perfect for that job. I've upgraded it, and it's still working fine, so I don't need to buy a new one yet.
My main desktop I do everything else with is Linux, and I have a small Windows test box to.