being a predictor of economic success. It is not the only thing that affects outcome, but it is the one that is most corelated.
So many people say IQ tests don't/can't really measure intelligence. Well, we do know there are a lot of smart people out there and this is the best we have come up with so far. Real world results of individuals match well with what we would expect. I think most people don't like where they rank and are probably biased against the results. If you are quite smart, and are in the top 25%, you still probably want to feel more special than whatever rank you got.
We know they are pretty accurate (not perfect) because they can get multiple sets of questions, and almost all the time, if you rank high in one, you rank high in other sets of questions. Not always exactly the same place, but someone scoring in the bottom 25% doesn't just jump to the top 25%. You are basically always going to fall within a few percentage points on other sets of questions.