They do that to create a "balance" of water in the black tank. Lots of folks are super conservative with use of the toilet water. "Girl Campers" use more toilet tissue (every flush). The idea of bath sink water into the black tank is suppose to add a bit more water, creating more dilution. I've never told my wife about it. I don't want her trembling during face wash about excessive water use. With just the two of us and huge tanks it hasn't been a problem yet. Once we head out West and start boondocking more...we'll have a "class" about it. Three campers back was a Lance Truck Camper an she well understood "small tanks". Graduating from that to the Chateau, she thinks there's no limit now!!
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie