Door Hangers are primarily used to safeguard the privacy of guests in hotels, resorts and inns as it allows housekeeping and hotel service team to stay out of room without disturbing the guests. 

Door Hangers can also be used as a promotional tool by announcing offers/ discounts and special product features by handing them on the doors of flats in large gated communities. It is a subtle and elegant way of informing your target audience about your product / service.