Well all the old terminology is the same the only reason the number system came about was so the session players could translate keys without re-writing anything during the tracking phase when artists change keys at the last second while laying down a scratch vocal. The whole idea was for it to be exactly that. Simple. I don't know why a band would necessarily make it mandatory to know but it does help us out when we record a typical 5 song session in 3 hour blocks.
I believe it's more so for playing live in the event that not all members are familiar with a song. Someone can guide the others with just a hand signal, right ?
