There is a lot of different possible applications. Check the status of equipment, keep track of temperature in each room or apartment. Spot water leakage as well as securing if someone opens a door (access control).
Improve your building management (BMS); analyze the values of fluid meters (electricity, gas, water) and control temperature, humidity, lightning etc. Monitor floors and internal pipelines and avoid severe leaks. Integrate all to your SCADA, BMS or GIS system.