"Dummy hardware" is used on doors that are not opened, but the door knobs are just there "for show". This type of hardware is common on double doors where one door is stationery. Some double doors have door levers where one side opens and the other side has the "dummy hardware" so it looks uniform with the other door.
Dummy door knobs can also exist on doors where the latch is located somewhere else on the door besides the door knob. For example, an outside door or gate may have a latch somewhere else on the door, but have a dummy door knob to make it look more appealing.