I can't remember if this topic has been discussed on here before or not. Do bag liners such as this really add more warmth to the bag? Or they just mostly keeping human's body oil from seeping into bag?

I was thinking, if I was to take my sleeping bag (the 15 degree one) somewhere cold and the temperature would be right around the bag's comfort rating. And say the temperature drops 5 degrees below comfort rating, would a liner really boost the warmth? I wouldn't expect a 40 degree bag compared with the liner I mentioned above to keep me warm in 20 degrees weather, but a 15-20 degree comfort rating in say 10 degree weather I wondered if it could make a difference?