You can control it from anywhere so if you're coming home earlier than normal or know you need to pre-cool the house because you're having a party later you can do it from your phone. Or just being able to control it from anywhere in the house, this can be done with X10 or something but that requires additional hardware to bridge between the X10 network and the internet. Most IoT devices are similar and use wifi and a central server because it's the most convenient option for configuration, with both sides hitting a central API you don't have to worry about ensuring clients discover all the devices on the home network and it's then trivial to allow control from outside the home.