It can be useful if you need to make a decision on how active to be outside. It’s usually not a good idea to be running/cycling (for fitness) or other similar activities outside with very poor air quality.
So if the air quality is bad, you'll drive. I see, very useful.
Or not run outside. Or if I cycle, not press hard.
A few days ago it smelled and looked like smoke in San Francisco. A “Hey Siri, air quality” into the Watch returned “Unhealthy.”
So you don't trust your own eyes and nose and need "Siri" to confirm it for you?