The only reason I can think of besides app testing would be if you had a Mac released in late 2013 or early 2014 and you wanted to run macOS 10.9 directly. You

The only reason I can think of besides app testing would be if you had a Mac released in late 2013 or early 2014 and you wanted to run macOS 10.9 directly. You can sometimes use Internet Recovery to re-download a copy of 10.9 as it shipped on the computer, but maybe your Internet is poor, or you want to install the OS from a different model of computer, then put it back in the computer that originally shipped with 10.9? So there are reasons, but the list is pretty small. You can see which OS was originally installed when the Mac model was first released at https://everymac.com/systems/by_capability/minimum-macos-supported.html https://everymac.com/systems/by_capability/minimum-macos-sup... And I should have clarified that there's a difference between doing an Internet Recovery and a Recovery Boot. Your Recovery Boot is often the latest version of macOS (though I might be wrong about that, all I remember is the UI changes with new version upgrades) while Internet Recovery is the original version of macOS that shipped with your Mac. (It always looks a bit ugly...)