Giving preferential treatment to legacy students is unpopular. It strikes most people as unfair. It’s often framed as mediocre kids getting into top schools because their parents bought them their way in.
But what’s the point of college admissions? What are they trying to solve for? In general, a college wants to let kids in that will thrive. At the bare minimum, they don’t want to let a kid in just for her to drop out, especially if its an expensive elite school. Similarly, the school doesn’t want kids to do poorly in classes or feel left out socially.
Everything they look at is a signal for how well the kid will do in school. Much of college is taking tests. So GPA and SAT scores, which mostly measure the kids ability to take tests, are a decent proxy for how well the kid will do in school. curriculars are also useful in that they predict how a kid will adjust socially.
For instance, Princeton legacy admissions are actually pretty smart. Only 2.2% of legacy admissions scored below 1390 on their SATs, while 12.8% of non-legacy scored below 1390.