[andrewaylett]

Kids of today, etc.

AT power supplies didn't have any mechanism for the system to tell the power supply it wasn't needed any more. So when you shut down the computer, it would wind up at a screen with a message approximating "it is now safe to switch off your computer", at which point the system would halt.

ATX power supplies added the ability for the OS to trigger an actual power off. But that's a different end-state to halting, and if you halt the system then it stays on. You may wonder why anyone would want to halt when power off is an option, and to be honest I'm not entirely sure -- possibly because you have a hardware watchdog which will trigger a reboot of a halted machine but not of a powered off machine?