(a)The number of persons originally enlisted or inducted to serve on active duty (other than active duty for training) in any armed force during any fiscal year whose score on the Armed Forces Qualification Test is at or above the tenth percentile and below the thirty-first percentile may not exceed 20 percent of the total number of persons originally enlisted or inducted to serve on active duty (other than active duty for training) in such armed force during such fiscal year.
(b)A person who is not a high school graduate may not be accepted for enlistment in the armed forces unless the score of that person on the Armed Forces Qualification Test is at or above the thirty-first percentile; however, a person may not be denied enlistment in the armed forces solely because of his not having a high school diploma if his enlistment is needed to meet established strength requirements.
An AFQT score is derived from the ASVAB(essentially the militaries’ IQ test). IQ scores are based on a normal distribution of scores from the general population with a mean of 100 and a standard deviation of 15. So the 30th percentile represents an IQ score of 92 while the 10th percentile would correlate with an IQ of 81.
To some extent you probably get more generally intelligent from practicing IQ tests in the same way you might get more generally intelligent from stretching your mind in any way. However, the increase in IQ score you achieve after practicing for the IQ test is (just guesstimating because there’s obviously no studies on this) >90% due to learning the patterns of IQ tests and <10% due to increased general intelligence as a result of studying.
To answer as to why IQ is helpful, it’s useful for making conclusions about how different factors influence intelligence. It’s more difficult to prevent lead from poisoning people’s brains when you can’t conclusively say how much it’s poisoning. Supposing all the people with low IQ scores due to lead poisoning practiced for the test to make themselves feel better with a higher score, their studying would muddy the stats and make for weaker arguments on the side of those wishing to ban excessive lead. IQ is also relevant to certain diagnoses, such as for the diagnosis of ADHD where a deficiency in working memory and processing speed but not elsewhere supports a diagnosis.
In terms of whether IQ / intelligence is 100% genetic, obviously not, I don’t think I said anything that could even suggest that. I’m not an expert so I’d appeal to this link for specific answers. Just skimming it seems to suggest anywhere from 50% to 80% heritability of IQ, although heritability as a concept is kinda unintuitive and hard to apply to everyday things.