At first glance from the hall, their function is disguised behind the appearance of wall ornamentation (the handles are the only giveaway). Conventional norms around service in many places in China seem to revolve around the belief that less interaction with the server is the ideal.
These doors are meant for keeping the flow of tea and food steady and predictable, while minimizing the need for outsiders to enter the room; the tables in front of the room-side of the doors hold empty teapots and dishes – both are handled by staff whose job is to run hot water for tea and dishes to/from the kitchen. In use, the door has its own signal language of sorts, being propped open if there’s something needing picking up.
Inside, pretenses of comparable camouflage are dropped. Diners are meant to know that there is a door in the wall, that their food will be coming through it, and, yes, that smartly-uniformed fellow/miss standing off to the side of the room trying to channel a piece of furniture is in fact yours for the bossing-around if you so choose.
A door as an indicator/amplifier of a context’s “ideal”.