An illustration of winter heat flow and the effects of different levels of insulation.
Winter Insulation Benefits
In the winter, heat from inside your warm home flows through the walls, windows, and attic out to the cold outside air.
The heat loss to the attic is significant because of the fact that heat rises. The air near the ceiling can easily be 10 degrees warmer than the air near the floor in the winter time. The heat flows to the attic quickly if there is no insulation to slow it down.
Despite the constant heat flow into the attic, the attic remains cool because the roof is not insulated. The result is a constant drain of heat from the house, which means that you constantly have to pay to put it back in (using your heater).