The 256 bytes is the boot loader? You do not need much for that. Basically enough to init the cpu, setup a couple of interrupt tables, and kick it running. For example the 80s/90s version would be the first sector on a floppy that inits the OS. In addition to the BIOS setup that probably put the read of that floppy in motion.
If I remember my GB history right this was a DRM measure to stop companies from copying the rest of the game rom and it still work on the actual hardware. Most emulators did not really need it as they had basically figured out what that area was doing anyway. Think in the GBA they even had an animation in there (which you can tweak going by setting a few HW registers) and the ROMS checked to see if the screen memory was correct at startup.
If I remember my GB history right this was a DRM measure to stop companies from copying the rest of the game rom and it still work on the actual hardware. Most emulators did not really need it as they had basically figured out what that area was doing anyway. Think in the GBA they even had an animation in there (which you can tweak going by setting a few HW registers) and the ROMS checked to see if the screen memory was correct at startup.