Beekeepers are told to put an entrance reducer on each hive before winter starts. Oft citing the need for an entrance reducer is to keep rodents out. It certainly does do that, but that's not why we use them.
In beginner beekeeping kits, included is a thin wood bar with several notches removed. One large gap and a small one, confusingly notched on separate sides of the bar. They are meant to minimize the hole where bees come and go from, giving beekeepers more control.
But what does a beekeeper want to control with an entrance reducer?
The population of a hive naturally ebbs and flows throughout the year, creating an ever-changing work force of bees responsible for guarding the hive. Decreasing as winter looms or the colony is being weakened.
Whole colonies can take a turn for the worse quickly and new colonies are being started yearly, because of this, beekeepers are often supporting weak or small colonies. As the bees keeper, we have a responsibility to support them as best we can.
For these two reasons, we use entrance reducers year around. To be proactive in case of decreasing populations and to protect weak, small, growing hives. Only removing entrance reducers during periods of high nectar flows concurring simultaneously with a large colony. When we see bees struggling to find a way inside the entrance because too many bees are coming and going, that is typically the only time we remove the entrance reducer.
Less of a hole for bees to guard, means less bees dedicating their days to guarding. For every bee that isn't a dedicated guard bee, this is one more bee that can be foraging or responding to other needs the colony may have.