"IQ" is just a terms for something on the map. It's what we measure. It's not a platonic idea. It's a mistake to treat it as such. On the other hand it's useful measurement. It correlates with a lot of quantities that we care about. We know that because people did scientific studies. That allows us to see things that we wouldn't see if we just reason on an armchair with concepts that we developed as we go along in our daily lives.

Scientific thinking needs well defined concepts like IQ, that have a precise meaning and that don't just mean what we feel they mean.

Those concepts have value when you move in areas where the naive map breaks down and doesn't describe the territory well anymore.