The message
from "Tim Ward" contains these words:

But someone must have invented it for some reason.

UKP and USD have been around for donkeys years in usenet and email.

I thought the reason was

A) to distinguish UKP from USD, for systems which cobble up £ and $ signs,
B) for readers who don't notice the difference between £ and $
C) to distinguish UK pounds and US dollars from pund and dollar
currencies elsewhere.

Janet