At the most rounded end of the egg (obtuse pole) is the air chamber (necessary for the chick to breathe during the hours before hatching) which increases in volume as the days go by. The further away the laying date, the greater the volume of the air chamber.
If the eggs were stored with the point (acute pole) facing upwards, as the air chamber increases there would be the risk that, behaving like a real air bubble in a liquid, it would tend to rise and could touch the yolk, contaminating it. The albumen is less likely to be contaminated than the yolk, because it is alkaline and bacterial proliferation is more difficult.