why is there a dual element bulb in

there is 2 places for current to be carried to the bulb in that socket. one of the terminals in the socket provides +12 volts the other provided ground when the door is open and the switch is grounded out. most bulbs get their ground from the base of the bub but that wont; work here cause the door switches provide ground when the door is open you need a #1004 bulb for the dome light