Although you’ll get two layers of waterproof protection with jackets that use internal membranes, these internal membranes have holes because they need to be breathable, and allow the water vapor from your sweat to escape outside the jacket so you stay as dry as possible even as you perspire. While the water vapor can come through the holes on the internal membrane to exit the jacket, water droplets from the rain are much larger than this and can’t permeate the holes on the internal membranes.