Most modern 17/18 wheels list the offset and not the backspace, so knowing how to use both is very handy. It's just a different tool, both have their uses, pros and cons. If you understand what each measurement is telling you then there is a time a place for both specifications.
I honestly don't have to calculate anything most of the time, I know the mm to inch conversions in like 1/4" intervals and the breakdowns of offset to backspace and backspace to offset in my head for a most of the A-body wheel sizes anyway.
If the US would just ditch the archaic Imperial system and adopt SI units all of this would be easier. For most folks it doesn't make much difference in the day to day, and for anyone that does anything with math or science the SI system is about a million times easier to deal with.