I remember from the owners manual of my E36 about 20 years ago it stated the reason for having this was so that you could blow cool air to the face to help keep you more alert, without making the ambient temp too cool for everyone else. It works very well at that.
I use this feature all the time on my M3 (and all my past BMWs) but really miss it when driving my wife's Audi that doesn't have it.
2017 F30 340i M Sport/6MT - Tanzanite Blue/Saddle Brown (mine)
2013 E70 X5 3.5xdrive (hers)
2011 E90 M3 6MT (sold)
2005 E46 330i ZHP 6MT (sold)
2001 E53 X5 3.0 PP/auto (sold)
2000 E39 528i Sport/PP/5MT (sold)
1998 E39 528iA Premium (sold)
1997 E38 740i (sold)
1993 E36 318is (sold)