The sensor will tell the thermostat that it is 73 in that room and the HVAC will kick on until it gets to 72. On the reverse side, if you have a room that is always hot but no one is ever in it, you ignore the sensor and the thermostat won't kick on the HVAC no matter how hot it is in that room. You can basically only tell the thermostat to pay attention to the rooms that people are actually occupying all the time, therefore theoretically, it saves energy by ignoring rooms that arent' used. Also, when there hasn't been any movement in the house in a while while there are supposed to be people home, the thermostat will assume you're away and go to away mode which generally has warmer allowable temps in the summer and colder in the winter ----therefore, saving you energy.

You can always close your vents in unused rooms too in order to conserve energy along with ignoring the sensors in those rooms.