Could be handy for boondocking. You could just hook up a hose from it to your municipal water inlet, but it would probably be better to drain the ERO tank all at once into your fresh tank after you've used up the water in the fresh tank. That way you could shut off the ERO's water pump and put the ERO tank awayŚ or go refill it somewhere.
I don't see myself needing one for my Airstream. But if I was still working for a living and didn't have the luxury to evacuate for every hurricane like I can now, I'd consider buying one for a home hurricane kit. Probably more sanitary than the usual method of filling the bathtub and dipping water out as needed for other purposes!
Engineering: Finding complex solutions to simple problems you didn't even know you had.