Noise cancelling headphones can cancel noise without any music playing.

It's not 100%, but they'll get rid of like 80% of the background noise without playing anything. To get that last 20% yeah you need something playing to help mask it(even at an extremely low volume).

Sometimes I wear mine with nothing playing while at work just to get rid of random office chatter.

You can use them to tune out noise around you. It muffles conversations, keyboard clacking, people sniffling, airconditioning noise, etc.

Seems you failed to realise that 'noise-cancelling' headphones is good for noise cancelling, not just listening to music.

My Bose QC35 set is ~4 years old, on their second set of muffs and only play sound when I fly, once or twice a year.

You can get a nice pair of ear muffs for much less.

But the parent was also talking about ambient stuff. Noise-cancelling headphones would definitely help.

They're cancelling noise whether or not you're listening to music ;-) Sweet sound of silence.