We had one on our first motorhome (owned for 5+ years) and never used it. It was just an extra spigot to empty out and winterize at the end of every season. I think it's one of those features that some manufacturer thought to add as a "new and innovative" design at some point (late 80's or early 90's maybe?) and other RV makers grabbed onto the idea because it was cheap to install during the building process... despite the fact that it was probably never used much by customers. Especially when it was a new concept, full hookups were a lot more rare and I can't imagine many people would waste a lot of fresh water by spraying it onto the ground.
Best carpet sweeper