Fleece fabric is traditionally used as an insulating layer, available in a variety of weights it can be used as base to mid or outer layers.
Fleece is used as part of any layering system in that it will help transfer moisture away from your body and trap air helping to keep you warm. Using an outer shell completes the layering system giving protecting wind and rain.
Traditionally fleece looses its effectiveness if not protected from wind by an outer shell. Developments of fabric now include fleeces with membranes than can make garments both wind and waterproof.