Colonies without entrance reducers frequently keep their brood nest in the second hive body and pack less fall honey into the brood nest. Entrance reducers make it easier for bees to regulate nest conditions in the bottom hive body, and early placement helps the bees prepare their nest for winter. On hot days in September, you may observe bees hanging out on the front of their hive when entrance reducers are in place. I have not found any evidence that this is harmful to colonies, and the improvement in nest organization is a clear advantage. In addition to a reduced bottom entrance, colonies need an entrance near the top of the hive during winter.
Bees consume honey to generate heat.
Carbon dioxide is heavier than air and it settles to the bottom of the hive and flows out the bottom entrance. Water vapor rises from the cluster. In hives that do not have a top entrance, the water vapor hits the cool surface of the lid, condenses and drips on the cluster. Top entrances allow water vapor to escape. In addition, bottom entrances can be shut off with dead bees.