Presumably for marketability, Harvard wants a small percentage of lower and middle SES students in order to make it appear that it is not exclusively a school for the scions of the wealthy and well-connected, even though it mostly still is (57% of undergraduates do not get FA grants of scholarships, which Harvard’s net price calculator suggests needs a $2xx,xxx family income, and only 11% get Pell grants, meaning from approximately bottom half of family income in the US, according to https://nces.ed.gov/collegenavigator/?q=harvard&s=all&id=166027#finaid ).
Although preferring a strongly upper SES skew is not what they will publicly say, it is likely to be desirable to them, since (a) it means less FA cost, and (b) elitist employers probably prefer those from upper SES upbringing, and those from lower and middle SES backgrounds become more acceptable to those elitist employers after four years of socialization in a predominantly upper SES environment.