The history dates back eons to first and second class ratings (cf command and copilot for heavies) which, I suspect, were introduced early during the two airline policy days to make it a tad more difficult for F/Os to do a bunk and head off overseas - as well as saving a few minutes and dollars on the re-rating work.
There was also a class one rating (cf command for lighties).
These then were subsumed into the command and copilot ratings.
The end result is all sorts of archaic ratings in our licences for not all that much benefit ?