The switch on the 'dash board' is an accessory added after it left the factory .
Sounds like somebody went to a lot of trouble to wire the lights in, I wonder if the lights are the factory kit that was available for a while ?
I see nothing in the wring diagram about it .
It may have been a European requirement at one time, is my best uneducated guess .
I added the switch when I installed the PIAA 40 driving lights on my '81, but I chose the wrong power wire, I can only use the driving lights on the headlight low-beam position, I used a wire off of the 'spare' electrical connector under the seat, usually covered with a light blue cap .