The reason for a Queen Excluder is to allow worker bees to pass through to the honey super without letting the Queen. The cost effective way to keep larvae out of your honey.

The purpose of a queen excluder is to ensure that the queen does not lay eggs in the honey supers. This ensures the honey within these supers stays clean and free from eggs/larvae. Queen excluders are placed in between the brood boxes and the honey supers. They are designed so worker bees can pass through the screen but the queen cannot as she is too large to fit through the slots.