A duvet is like a slipcover or a pillowcase for a comforter. It can be highly decorative and need to be dry cleaned, or simple and machine washable.
We use a pretty, but machine washable one with our thick down-alternative comforter. We don't use a top sheet or blanket, so I only have to shake the comforter out to make the bed. We also tend to tangle sheets and thin blankets, and I end up cold in the middle of the night.
On wash day, I just remove the duvet and wash and dry it (bottom sheet and pillowcases too).