why is there a dual element bulb in

I believe on the older ones the dual filament socket/bulb was for operation at the door switch and the dashlights switch. For example.....open the door the dome light comes on.......Turn your dash lights all the way up, then there's an extra spot on the switch to turn on the dome light. The dual element is just like a stop/tail lite bulb, it allows for dual operation. Don't quote me but that has been my experience.