Please tell me one "practical purpose" were "Intelligence is largely genetic" as a use...

If we know intelligence is largely genetic and differs across genetic phenotypes, it would tell us that public policies designed to alter group outcomes by altering the environment might be counterproductive.

To take an extreme example where genetics is the clear cause, cats are underrepresented in college. Because we know that cats are genetically less intelligent than humans, we can conclude that affirmative action/college for cats education programs/etc are a waste of money.

Another use would be knowing that if only 50% of the population has the intrinsic intelligence for college, college prep programs should not serve more than 50% of the population (modulo technical details).

Off the top of my head: eugenics, diverting the smarter parts of population to science and leadership roles.

Not saying it's moral, but definitely and obviously practical.

This might be practical, but it does not make for a proven profitable use of the "fact". Currently it is way easier to test children/students/workers for intelligence or academic achievement rather than subject them to a genetic test or pedigree inspection.

Pedigrees and genetic testing in animal breeding are way more useful. But there you have completely different structures and goals. And some of them, particularly diseases with high heritability like canine hip displasia, can't be solved by current genetics yet.

It's useful in convincing parents to stop wasting their money on Baby Einstein CDs and other unscientific marketing ploys.

Would telling them "Your child is stupid because you are stupid!" really be more successful than pointing out a lack of effectiveness? And would they really ignore the left-over variability? And will they stop at Baby Einstein CDs or will they just neglect any IQ improvement method, like regular and complete school attendance?

You're missing the point entirely. Imagine if people thought the same way about height as they do intelligence. Neurotic parents would waste all their time trying to make their children taller. The nightly news would be filled with segments on how X or Y impacts your child's height. Yet, none of this would do much to change outcomes, because height, though highly variable among siblings, is mostly genetic.