To get the latest images off the card first. Aperture allows any form of sorting before ingest, this can be as simple as capture time, but a click of the triangle arrow can reverse this so that the most recent image is ingested first.
This would be handy for when you have a lot of images on the card, but the most important and time critical images are the last ones you've just shot. If the images could be ingested in reverse order then there would be no need to tag the files (sometimes there isn't enough time for that).