Stem Exports are becoming are much more frequent requirement in all sorts of areas from music production to film post and even for theatre or live backing tracks. This last scenario was the case in point that made me go looking for this feature.
I was working on a show where the MD produced some excellent backing backing tracks which were well mixed and balanced but of course there is no substitute for hearing a track in the venue itself and adjusting your mix accordingly. This is even more relevant for live backing tracks than complete recordings because, of course backing tracks will only ever be heard in the venue, so you may as well mix it for that venue. Given the time constraints on most productions performing a full mix would be impractical, but given yourself the option to adjust kit against rhythm section say, or tweak the level of backing vocals makes a lot of sense. It also makes for a more enjoyable show if you are mixing it yourself!