You can have a much higher quality material for both inside and outside, and washing the cover is easier than a full comforter. You wash the inside more rarely with a duvet cover. You can also have one inside and many covers for variety of styles during seasons without having an entire room of the house for storing comforters
Vacuum bags are good for storing that kind of stuff compactly. My duvet and duvet covers semi-permanently live in one! The only kind of blanket I’ve found that is not too warm is gauze.
