Why "plug in computer," from High Sierra Installer?

High Sierra requires that a firmware update be accomplished before installation. Firmware updates require the power connected to prevent power failure while the update happens. If there is a failure while it is updating, it might make the Mac unusable.

If your battery is malfunctioning, it will interfere with the Mac detecting the connection to the power supply.

If your battery is removable, remove it and leave the Mac plugged into the power supply.

Exactly my point. The warnings for plugging in the computer during installation of Mac OSes appeared in the 2000s, but still would let the installation proceed at the user's risk.

You didn't seem to understand. All the previous OS installs did not include a firmware update. Firmware updates have always required the Mac to be connected to power because of the danger. There is no similar danger for an OS upgrade.

Wow! Serious stuff. How many times in the future do you think an otherwise "normal" upgrade will include the dreaded "firmware" update that has never happened in the history of computing? Us users should be warned!

Probably as many times as Apple finds it necessary. So, far it only appears to have happened once or twice.

Usually, firmware updates are distributed separately, and they only affect certain hardware. In this case, High Sierra's changes required a firmware update for every Mac.

I assume this one was because of the new File System, APFS, allowing the Mac to boot from an APFS drive.

if a firmware update dies in the middle, your Mac could become a "brick" -- completely unusable.

It would likely require a mainboard swap to become serviceable again.