Same reason a ceiling light does.

I know, I know, you're thinking that doesn't either, but you can control it remotely through an app or website, and automate certain actions. You might want your air cleaner to start running when you leave your office, for example so that your air is dust-free when you get home.

I've actually used a Xiaomi light remote-controlled over the internet to simulate being home while on another continent so that anyone casing the place for a burglary might be dissuaded. I disabled its internet connectivity when I was done with that.

I wanted to switch it off automatically at night. Doesn't seem to be necessary though so haven't connected it