In my case, I'm an embedded dev. So some of my own code is doing memory-mapped I/O to hardware, inline microcontroller ASM, or other stuff that can't physically

In my case, I'm an embedded dev. So some of my own code is doing memory-mapped I/O to hardware, inline microcontroller ASM, or other stuff that can't physically happen on the system running the unit tests. So mocks (or fakes, it doesn't matter much here) are necessary to avoid running hardware-specific code on the wrong hardware. Of course I try to make those fakes at as low a level as possible, but there's still a substantial part of my code that can't be unit tested, and needs test programs in the actual firmware to check it.