A bonus of the thermostat is that it will light up when someone enters the room. So when you get up at 2:30am to use the bathroom, the thermostat will not only show you the time and temperature, but will also light your path, so that you aren't waking anyone up by turning all the lights on.
Nest Protect - Smoke and CO2 Detector
One reason to use Nest products is that they work together. If there is a fire in your home, it is common for the heating system to leak carbon monoxide into your home, or spread the smoke more rapidly. In this case, the smoke detector automatically shuts the thermostat off, thereby helping to keep the threat from spreading.