Smart thermostats are what make them really good. In climates where it gets really cold, you need to have a way to switch over from pumping cold air to pumping hot air to using a secondary heat source.
It wouldn’t have to be that smart, or even networked. A z80 or 8008 processor would have been plenty. High end systems could have done it in the 70s, with costs coming down quickly through the 80s.