Okay, you know how we put pillows in pillowcases, so that when it's time to wash the bed linens you don't have to wash and more to the point dry the pillow stuffing?
Take that same concept and apply it to your comforter or bedspread.
A duvet is a thick blanket that's designed to come apart so you can wash or exchange the outer layer without having to wash the fluffy insulation. Another feature is that you can own multiple covers and one actual blanket, so if you want to change up your colors you can just swap out the cover. One-piece comforters don't fold up that compactly for storage, but empty duvet covers do.