Having a monitor is easier than constantly reflashing for every little change. (back in the days of EPROMs, much easier)

fun?

That was the primary reason - a fun weekend project converting 6502 Assembly to x86-64 line by line. The x86-64 version comes in at a 384-byte binary due to instructions being a bit longer.

It is wildly impractical to not have an OS and have to manually program your computer every time you boot it. But don't you want to try it?

That depends. Do you want to avoid the Thompson back door? Maybe a 6502 or an 1802 with flippety switches and blinken lights can be verified not to be compromised.