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.
link
Two quotes:
"But let me say on this issue, if we got to know each other, you'd know the Pences love everybody," Pence continued. "We treat everybody the way we want to be treated. But on this issue, and it's frankly something that when the Obergefell decision was made which legalized same-sex marriage in America, the Supreme Court, Justice Kennedy wrote at the end, that this decision will likely create an intersection and tension between people in same-sex relationships and people in the exercise of their religious liberty."
"Look, I believe marriage is between a man and a woman, I believe in traditional marriage, and I believe marriage was ordained by God and instituted in the law, but we live in a pluralistic society, and the way we go forward, and the way we come together as a country united, I believe, is when we respect: Your right to believe and my right to believe what we believe," Pence said.
There's not exactly a specific law because it was decided by the Supreme Court. He'd most certainly support a law in Congress to overturn the Supreme Court decision though.