In my experience SAT scores correlate pretty well with intelligence (to a point), which is why they exist.
It's more like ... a series of imperfect correlations added together. Someone who got an 800 verbal is x much more likely to write poetry. Generally speaking, you'd rather hire the kid who got a 750 than the one who got a 600. Maybe the 800 math scorer is slightly less likely to need a calculator in daily work at the lab than the one who got a 600. Add the small things up, and the overall score is, I'd say, pretty helpful. Not perfect -- and probably not as ironclad as it used to be treated by university admissions officers, but still helpful. Of course, this is all coming from someone who thinks this is the worst Supreme Court decision since 1857, not because of the immediate affirmative action it encouraged, which was probably necessary, but because of the insidious idea that was planted in the labor market: you are not allowed to explicitly discern an employee's intelligence before you hire them.