They have names and educational backgrounds to hand, and it's well known that all things being equal, tutors would always rather go for a kid from a state echool / underprivileged / non traditional background over one that went to a top public school.
It's not just a matter of hitting targets - the alternative is that everyone who gets in is from a wealthy family or went to a top fee paying school.
If everything is blind, you get less of an opportunity to engage in this kind of affirmative action.