Because that's what it really is in case of the Bermuda-based Models
76/77 - an IML partition.
> Wouldn't the system fail to boot if it couldn't find the initial
> microcode to load?
Yes, there's no fallback Stage 2 code in the Bermuda ROM, so if it can't
find a compatible IML image anywhere (floppy, IBM SCSI) it will print an
error and then halt.
> I've run the systems without the convenience partitions or hard drives.
> So ... why IML?
If there's no IML partition you have to feed it the reference disk and
it will load the IML image from there. Once that's done, you can load
your OS from another floppy (the IML image will stay in RAM until you
power down the system or force cold boot in some other way).