A person releases moisture at night. Sometimes half a litre, sometimes a touch more. That moisture has to go somewhere.
In a synthetic duvet
In a synthetic duvet it lingers. The filling takes no water in. The moisture condenses against the fabric. In the morning the bed feels clammy. After a while odours arise, and dust mites find there a warm, moist habitat.
What down does
Down does it differently. Every cluster of down absorbs moisture into its fibre. During the night the duvet holds it. By day, when you leave the bed open and let the duvet breathe, the filling gives that moisture back to the air.
The result
The result: a duvet that stays fresh for many years, and a bed that does not feel in the morning like the one you just stepped out of. That is why opening a window in the morning matters more than it seems. You give the duvet the room to finish its night-time work.