IQ testing was originally devised by Alfred Binet with the intent to identify children who needed more help in school, but unfortunately about no one else took that view and instead it became a means for discrimination and elitism.
Studying and measuring cognitive ability might seem like an interesting area of study, and it really ought to be, but a lot of the research is rooted in early 20th century elitism and classism, and the narrow-minded value-laden nonsense of the era.
You can probably see how the very nature of this topic is going to inevitably lead to it being a propaganda tool to justify inequality and hierarchy. It’s right up there with economics as dismal science, in my view, and it’s no coincidence that IQ is fetishised so much by reactionaries.
There’s a pretty good book called What Intelligence Tests Miss: The Psychology of Rational Thought, which delineates intelligence (at least as measured by IQ tests) from rationality. Basically, there’s no correlation: you could have an IQ of 200 and still make bad inferences and irrational decisions.
IQ as a predictor of “life success” is regularly touted, but it’s pretty flimsy and other traits are far more important factors (being a charismatic liar is far more useful, for example.)