I know how the game works, I've paged through the Pearl book. But here, in this case, I don't care much about causality. I can observe the existence of stupid people and smart people (and somewhat-stupid, and middle-of-the-road, and a bit smart, etc.). I can roughly rank them on the smart - stupid axis. That axis won't capture all the diversity and the variation, but it will capture some. Whether what it captures is sufficient depends, of course. It depends on the purpose of the exercise and in some cases that's all you need and in some cases it's entirely inadequate. However in my experience that axis is pretty relevant to a lot of things. It's useful.

Note that here no prediction is involved. I'm not talking about whether estimates of g (IQ, basically) can/will predict your success in life or any similar stuff. That's a different discussion.