Most likely a "ROM emulator" that would plug into the ROM socket on the board, replacing it with a few kB of static RAM that you can pop stuff into from a computer.

In the mid-90s I used an already pretty obsolete CP/M machine to drive one of those for developing Z80 embedded code. Although PCs were around - 386s and even early 486s if you had the money, they offered no real advantage because the editor, assembler, and linker only took about a second to load from floppy anyway.

Given how much time I spend looking at 80x25 terminals I wonder why I put up with 20-second boot times now, over 30 years later.