The original Atari 400/800 included BASIC on a ROM cartridge.

To use BASIC, you plugged the BASIC cartridge into the system and powered up.

To boot something else (games...., from either cassette or disk) you first removed the cartridge, then powered up.

With the XE series, BASIC was built in to the console, so the "magic keys" were needed to tell the hardware to virtually "unplug" the BASIC ROM before it tried booting from any connected devices.