Elizabeth Bustamante @ snapeda.com compiled a list the top 10 humidity sensors available today. Most of the sensors on the lists are from Honeywell, but surprisingly the top sensor comes from Sensirion. She writes:
Air humidity is an important factor to consider when designing an electronic device. Having too much humidity in the environment can cause condensation and corrosion, which can lead to anomalies in performance or even failures.
To prevent this, engineers use humidity sensors. These devices are used in systems deployed in humid environments, such as industrial control, instrumentation, climatology, and agriculture applications.