Why? Can't answer that.
On a GM wiring diagram this wire is marked "(PVC+NYL)".
There are 3 places this wire is used.
Two in the number 40 circuit (fused but not switched) orange on the inside of the car. One side of the courtesy lights both the orange and white wire that grounds through the door switches, I think it is the passenger side it is not clear on the diagram. Why only one side? No idea.
The black wire to the under hood light (circuit 40).
The tan w/white stripe brake pressure warning switch wire to the proportioning valve (circuit 33).
I see no other wires marked like this. There are other notations like "(SXL)" on some of the under hood wires that actually cross the engine.