The first layer of attic insulation is sometimes stapled to the joists from beneath. The foil or paper acts as a vapor barrier. This prevents some of the moisture from the home from reaching the dew point and condensing inside the batt above the paper in a cold ventilated attic. It is a choice to eliminate or ignore the paper in the batt and staple up plastic sheet.
New tight construction with vapor barrier prevents high air exchanges inside the home, saving fuel bills. This makes high humidity in the home, sometimes requiring machanical air excahngers. However, it is a long-term savings to have superior insulation and have to open a window a crack or start the ventilator.