you have a single bus in a ring topology instead of a star network of wires coming from a central location. much less wire and with most indicators and even some headlights being LEDs the current carrying capacity of the +12V wire can be much smaller. GND is the metal substructure and the CAN (or LIN) bus is just two small gauge wires.

much cheaper and much less wiring needed if the bulbs (or bulb holders) can receive commands themselves.