Redundant nipple valve on the K3 or 3sz-ve engine block behind the catalytic converter

Earlier I was wondering what the redundant nipple valve, sticking out of the 3sz-ve engine block. Even daihatsu forum has no answer for this so I've searched and found the mechanic manual book for K3 engine and it turns out that is the cooling water drain plug. Not sure what is the common function of the nipple valve since the coolant drain can easily be done via the radiator drain plug and for the record that valve rarely being used by anyone in the lifetime servicing their K3 engine.