Here's the money quote: >> Importantly, these test scores better position Admissions to identify high-achieving less-advantaged applicants and high-achieving applicants who attend high schools for which Dartmouth has less information to interpret the transcripts. Precisely. SATs aren't about hurting the poor or disadvantaged. It's about giving them a chance.
IIRC most colleges correlate incoming and outgoing grades for specific high schools - so that a 3.0 average at a highly competitive high school (think Stuyvesant or Thomas Jefferson) is a better indicator of success than a 4.0 at a poor performing school - the SAT is very helpful to add context to a GPA for a students applying from schools where colleges don’t already have a sense for the HS program’s rigor.
I came to appreciate my high school education when I attended a highly selective university and found my grades going up from high school without actually doing any more work while seeing class valedictorians (I barely scraped by with a cum laude) from other high schools really struggling.