As chip package sizes get ever smaller, like the Microchip Tiny84A, in its 4mm QFM package, I just see it as a strategy to get more raw circuitry, and more functionality at the cost of multiplexing the pins or vitualizing by IO pin mapping in software.
There is a great article here:
I can only guess that it might be something like parts of the firmware being stored on the HDD that could get nuked by the Windows installer.
At least I have a Compaq DeskPro 486 that had a nice clicky graphical BIOS Setup utility (resembling current UEFI interfaces, completely out of place in the 90's) and that one was stored in a special hidden partition ...
Steve Ciarcia's SB-180 - as described in Byte magazine in 1985 - used a 6 MHz Hitachi HD64180 CMOS processor that was compatible with the Z80. Maybe a kit doesn't qualify as a home computer, though, but it certainly wasn't too portable.
(The company that Steve co-founded to make these kits — Micromint — still sells Z180-based SBCs, amongst other products.)