The holes are there because of how the bags are filled. The bags are
actually closed except for a small opening near the top that accepts
the feeder (basically a 2-3" tube). An operator will simply slip the
bag over this tube and the bag is filled with concrete. The weight of
the concrete will then push against the opening to close it. If there
were not holes in the bag, then the bag would explode while being
filled from the increase in pressure. The same technique is used to
fill large bags of dog food, cat litter, etc.