Who Uses IQ Tests?
The IQ test is more than a century old, about 113 years. Nobody knows what it measures in any fundamental sense. It has been and continues to be used for all sorts of practical purposes, some utterly frivolous reasons (membership in Mensa and similar deliberately exclusive clubs, and others dubious for yet other reasons. It's advantages are that it's cheap to administer and score as well as fairly strongly predictive.
The original purpose was classifying the mentally retarded, for purposes of planning their education. In one sense, that might be still the main use, to the extent that we are all retarded compared to somebody. The big breakthrough of IQ testing came when the Army adopted it for attempting to classify the hordes of WWI draftees. Because of the time and money the testing and evaluation required (in the pre-computer age), the Army wanted to know what it was getting for its money. Extensive evaluation yielded two important kinds of results: that higher scores were correlated with better performance in virtually every military occupation specialty, and that cetain minimums were required for acceptable performance in other skills. In many cases the Army spends a year or more training certain high skill specialties and there is a huge cost if the trainee can't due the work. Thus, if you got a high score, say 125, you might be trained to be an electronics technician, but if you got a 90, well the infantry always needs men.
Big employers test for exactly the reason. They want to make sure that you have a good chance of being able to do the job they are going to train you for. We know something about IQs in the NFL, for example. Certain positions, like quarterback and offensive tackle, require a lot of learning. You don't need a Charlie Johnson type IQ to be an NFL quarterback, but if you can't crack 110 your stock will go down. For cornerbacks, though, speed trumps IQ almost every time.
The SAT no longer allows itself to be considered an IQ test, but it mostly is. Exclusive schools tend to use this and similar tests to raise the prestige of their brands, though of course they might argue that they really just want to make sure students can handle their demanding course loads. This might even be true for a few schools like Caltech, MIT, and Harvey Mudd.
One of the more dubious purposes, I think, is admission to public school gifted programs. They are, I think, a required component of special education. Few parents, upon hearing that their kids had hit the magic qualification standard (IQ 130, two years ahead in reading or math in our local schools way back when) can resist the magical annointment as a wizard. These students get (or got) special counselors, lots of extr paperwork, an sometimes, the opportunity to take more advanced classes. I admit being among the guilty parents. This is mostly ridiculous. elementary school being a marathon not a sprint. Why not get rid of the counselors and use the money to hire teachers for advanced classes that any student could try?