My idea about the purpose of the duvet cover came when my hubby and I purchased a high quality goose down comforter many years ago. It was all we needed for covers - like sleeping under a cloud. Unfortunately, the ticking and goose down aren't supposed to be washed that often, if ever. Detergent and water strips the feathers and makes them less insulative. You put the compforter out to air, and shake it to redistribute the down. Then, we had a duvet cover to protect the comforter. We could take it off and wash as often as we wanted, like we would in changing the sheets. In fact it was our top sheet. It lasted a long time, before I had to make another for our comforter.