You don’t need one since the comforter already features a design or color scheme of some type. Duvet covers are usually used with down comforters to help protect them from getting dirty. Down comforters can be difficult to wash — which is why a duvet cover is a practical solution. But a comforter from a comforter set can be used with a duvet if you want to quickly change the look of a room, or prefer to sleep without a top sheet.