They are low cost, easy to install and robust enough to withstand the tough environments. This includes measuring moisture at line pressure, operating at high ambient temperatures or in hazardous areas.
They are mostly used in applications where measuring the water dew point in a gas is crucial to protect equipment or ensure the quality of products. To get the best accuracy from your dew-point transmitter you need to understand how the different types of transmitter work, and how each is suited to specific applications.
Michell Instruments' article ‘How accurate is your dew-point transmitter?’ explores this in detail.