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.

A few days ago it smelled and looked like smoke in San Francisco. A “Hey Siri, air quality” into the Watch returned “Unhealthy.”

AFAIK an AQI feature was actually first introduced in iOS 10, but only for a very limited set of cities/geographic areas (only ones I heard named directly were in China, which of course is both a big market and has had horrendous air quality issues they've had to grapple with). I heard it briefly vanished in iOS 11.0 actually before being reintroduced in a point update. I never had cause to use it myself back then.

Presumably in iOS 12 the feature is both more refined and Apple has also just plain worked to get more access to the necessary data streams and integrate them. From a brief glance around at some weather related APIs I've played with before it doesn't seem like AQI data, particularly sufficiently granular, has the kind of global support that is simply universal for other more common measures. I guess as knowledge of the health effects of small PM pollution and such start to become more widely known by the public, combined with worsening climate making it more common/worse in new areas we'll see sensors and data streams for that improve worldwide, at least in urban areas.

Nope; only on the new Series 4 exclusive watchface.

My Huawei phone shows AQI in the weather app (don't know if it's Google's or Huawei's app) but only for some cities – it shows up for Beijing and Shanghai but not Paris or Athens.

I found the Plume app to have good information and a nice UI.