"Smart devices" have chips, user-interfaces and/or sensors, and the ability to communicate over your home network.

The whole point of "smart appliances" is to collect data from your home. The same is true of "smart phones" and "smart watches". Appliance manufacturers are trying to come up with credible pretexts for connecting toaster ovens and light-bulbs to the internet.

One of the pretexts is remote control: "Siri, lights on!"

Devices hosting Siri are themselves spying on you, constantly reporting on your interactions.

The purpose is to collect data for AI (Artificial Intelligence). The AIs can interpolate from the data you provide all sorts of other facts about you.

Another purpose is for control. For example, some smart devices are already responding to central commands to themselves off when the power grid is in danger of overloading.