Anything that doesn't have a (reliable) safety interlock. Electric heaters or cooking appliances for example. Life-critical devices such as smoke alarms, etc (you wouldn't want a malfunction of your smart home controller or a malicious attacker to cut the power to your smoke detectors).

Generally, being able to cut power is a good thing for many devices. You shouldn't want to turn on dangerous devices automatically, but some of them might benefit from the ability to have power cut off.

The very first smart plug I ever bought was because one day I went on a trip and, after arriving from my flight on the other side of the country, I started to wonder if I had turned the coffee machine off. I had, which I confirmed with an indoor camera.

That coffee machine got a smart plug as soon as I got back. Others followed.