The Nest marketing I've seen compares it to a programmable thermostat. The idea is that instead of just having the heater or AC come on at regular times, it should behave in a more nuanced way. Your own patterns of turning it up and down matter, but so does the weather (it checks) and whether or not anyone is actually home.
It also now handles when the Nest is in a hot spot, for example a wall that a patch of sun crosses during the day. I was having trouble with that, we talked about it at length and shared historical temp readings vs other seven zones in the house, and they even sent me a new unit to make sure it was the sun not the unit. A few months later, the had an update for this issue.
