I too thought it might help the mass production process, my last car had a wiring loom that included connections for things not fitted.
But I do like the drain down idea as I still can't think what else it could be for.
My weird problem is that the boat basically has two states.
1. Winter, we drain the water tank using just the taps and turn off the power.
2. Summer we live on board for 6 months, power always on. Calorifier always on when we have shore power. Water stays hot even sailing x-channel.
The significance of that is we never have a tank with water in it AND the power to the pump off and there is always hot water in the calorifier
Until this year.
We had to rush home last month by ferry.
Tank nearly full, power off.
Came back to an empty tank and an engine bay and cabin bilges full of water.
Dried it all up, put water in the tank and turned the power on. We both heard a hiss but then nothing happened. No leak.
To cut a long story short, the end fitting on the pipe we are discussing had popped out to its first setting BUT bizarrely turning on the power caused the pump to burp, pressure to build up and it dragged the end fitting back to where it should be, closed.
I know this sounds crazy but I tried it probably 5 times.
Filled tank.
Power on.
Calorifier on.
All ok
Turned power off.
Overnight as the tank cooled water started to dribble from the end fitting. It took probably 12 hours for the dribble to start and that would have continued until the water tank was empty. The water tank gravity feeds into the calorifier.
I know that the hiss was the end fitting resealing as I actually managed to push it back on manually. It had definitely moved to the first position Whale describe in their fitting instructions.
So.
A bit odd.
My temporary fix has been to keep the power on as usual and raise the end of the pipe above the level of the calorifier by poking it behind pipes on the starboard side of the engine bay but I think an end of season job will be to either cut the pipe and fit a new end piece or remove it totally as per Nigel.