That valve is responsible for more "flooded trailers" than, IMO, any other device or failure. They are NOTORIOUS for failing, so to answer your question, It's the anti-siphon valve to prevent black tank contents and gasses from flowing from the black tank to your fresh water supply while a hose is connected to the black tank flush system.
They are "fragile" and can start to leak at any time, usually when you least expect it, and can fill the vanity with water before you realize what's happening. If you use it "as is" then I'd recommend always having someone with a flashlight watching the valve while in use. Alternatively, as Danny said, many (maybe most) people either replace it with a better, more reliable valve or just completely remove it from the system by installing a "homemade PEX fitting" and then install a "backflow preventer" on the backflush connector outside on the trailer sidewall. https://www.amazon.com/Homewerks-Wor...35478475&psc=1