An empty room has no scale. Buyers look at a photo of a vacant bedroom and cannot tell whether a king bed fits. So they assume it doesn't. That assumption costs you the showing.
Empty rooms also show every flaw. Scuffed floors, patched drywall, an uneven ceiling line. Furniture and rugs give the eye somewhere to land and make a room read as finished.
Then there is the feeling. Vacant houses feel cold in person and colder in photos. Buyers are buying a life, not a floor plan, and a staged room shows them one without saying a word.
We stage our own flips because the return is obvious at the closing table. When we started offering the same service to other sellers and agents, the results were the same: fewer days on market, more offers, stronger prices.
If your listing is empty, the cost of doing nothing is a longer carry and a lower offer. That is usually more expensive than the staging.