As someone who got into an "elite" university despite not coming from an "elite" background - the Legacy admissions are the reason people like me got the opportunity to integrate with people who otherwise wouldn't have given me a second glance. Being in a study group with a prince, going to a club with an heiress, or sculling with a billionaires' kid is not an opportunity most parents can give their children outside of sending them to a place like Harvard.
It's horrible that the world is so fixated on status, but you don't get to choose what game you play always - just how you play it. Ingratiating yourself with a Fortune 100 CEO's son can reap many orders of magnitude more benefit than being surrounded by randos who are better than him at calculus.
The value prop of a place like Harvard isn't that they're particularly good at teaching, it's the idea that, as a sufficiently smart and lucky regular person, you might have a shot at getting into an otherwise exclusive circle. No guarantees, and may people can't crack it (or choose not to, because it feels so slimy) even when given the opportunity - but Legacies are part of Harvard's value proposition.