I've never had one but I imagine it's like an Alexa or like Siri. It's connected to the internet for some reason and I think one of the features is that it can alert you if you leave the door open. What else you might need a smart appliance for I have no idea. And, everything I've ever heard is that the more bells and whistles the less likely you'll be able to repair any issues without it costing almost as much as to replace it.
Also, the very little I know about all of this stuff is that it's easier to hack into most things like this so you make anything else connected to the same wifi more vulnerable.
The ads are just one more reason not to ever consider one imo.