Because it was an easy way to make a junction.
The yellow is the return side of the dome (and courtesy) lamps.
If any switch on that part of the circuit closes, the path to ground is complete.
When the door switch closes (grounded), the circuit turns on the dome. The the dial is rotated, that closes the path to ground since the switch body is grounded, and the dome comes on. That's the way I read the diagram anyway.
