What is special about a "communicating door"?

The essential feature of a communicating door is that it enables movement between two rooms without this movement being common knowledge. A communicating door allows privacy/secrecy.

Technically, a communicating door is one that connects two rooms neither of which is a corridor, hallway, or anteroom.

That said, people normally use the term when they have two rooms whose functions are related, or the same, in mind, such as two bedrooms, two parlor rooms, two offices; or a study with an adjacent library, an office with a reception room, a dining room and kitchen, etc.

The concept must be relatively new: in ancient Rome, rooms were only accessible from hallways and most of them had curtains, not doors. Actual doors were few and far between. Romans (pagans and Christians alike) were far less inhibited than we are today. The obsession with doors and locks must be a medieval thing and may have something to do with poor hygiene and the attendant fear of appearing partially naked in front of others.

Communicating Doors refer to two doors that are back to back, usually with a shared door frame. These are predominantly found in adjoining hotel rooms that can be occupied by two separate guests or guests that rent both and want to be able to go between rooms without going out into the public hall - a room with parents and the adjoining room with their children for example. This does not refer to doors adjoining rooms within a suite. The doors can only be locked or unlocked from the room side but both sides of each door have opening hardware (a lever or knob).

Here is another use for communicating doors. We bought an old village house that had two sets of communicating doors. They have special door knobs to fit in the frame. One opens into one room and the other into the other room. That part of the house was used partially as a medical office and I believe the doctor opened the inner door when he was ready for the next patient or a patient would open it when she was dressed and ready to exit the exam room.