Humidity sensors are helpful for bathroom fans because they detect excess humidity in the air which activates the bathroom fan to turn on. This in turn helps to reduce mold and mildew. Using humidity sensors also reduces energy usage by automatically turning the fan on when it is needed.
Duct reducers are helpful for duct systems because they help provide a smooth transition between ducts of differing sizes. They help to maintain airflow which prevents disruptions that can potentially arise from different size changes.