Yes it does. The underlayment is just as important as the last layer of roofing. You will also loose the warranty on the roofing if all layers are not installed as per manufacture. All roofing membranes are systems which consist of multiple layers to water proof the roof correctly
Yes, the underlayment on any roofing product is the real protection that you get. The underlayment keep the water from penetrating into the house if it gets past the shingles. I always put down double layers when installing my underlayment.