What is the purpose of the moisture barriers found in structural firefighting personal protective equipment?

The moisture barrier found in firefighting gear acts to protect from the effects of steam. Firefighting introduces a massive amount of water into some very hot spaces and surfaces, and the resulting steam generated would quickly par - boil a firefighter if not for the gear we wear. Even with the gear, you can really feel it when the 300 degree floor level heat in a house fire changes instantly from dry heat to a sauna on steroids when you open that nozzle.