If its possible to compile & and sign iOS or MacOS apps that would be VERY useful in CI environments, but yeah hard to think of other killer command-line apps that only run on MacOS.

If they had good GUI support then some more things open up but nothing really essential. Garage Band has no equal in terms of ease of use, if you are into that sort of thing.

Might be useful to run ARM mac apps on ARM Linux vs using Wine + an x86 emulation layer.