The smart sensor is primarily marketed as an accessory for your thermostat, to detect presence in a room. In doing such it would help to regulate the air in that area of your home. The gotcha with this is that without a dedicated HVAC system for each room, it would simply run the main unit until it cooled off areas which the smart sensors detected as occupied. This means other areas might be too cold or too hot, depending upon the season, from running to cool warmer areas in the summer, or heating areas in the winter, that are occupied while the other areas got too hot or too cold. They do have the ability to detect environmental conditions in the rooms they are placed. But only if used with the ecobee smart thermostat.
The secondary purpose of these could be to act as a burglary sensor, to detect the presence of an intruder. I've not been impressed with my testing of this for that purpose.