However, if you have more than one thermostat (e.g. separate downstairs and upstairs air conditioning/heating) having a "smart" thermostat is awesome! I can change the temperature on both from my phone without having to go upstairs.

We also installed the additional sensors in various rooms in the house so I can control which zone gets used in determining the whole-house temperature (there's often several degrees variation between rooms in my house on hot summer days). The thermostats in my home were installed in terrible locations that don't reflect the temperature in places where people actually are so the remote sensors are fantastic.

Before the Nest, the bedroom would get way too hot at night because it was better insulated than the living room, making it difficult to sleep. Afterwards this was no longer a problem. Saved a bunch of energy too.

Other "smart" tech: remote controlled outlets, later swapped for hue bulbs, let me flip all four of my living room lamps on from the entry way OR the hallway. Before, there was no switch at the entry way leaving me fumbling through the dark for the wall switch at the other side of the room to turn the ceiling light on. Then I'd have to turn the floor/table lamps on/off individually.