From a college’s point of view:
- Legacy applicants are likely to be higher SES and lower financial aid need, so they help the college keep its financial aid cost down.
- Causing alumni to believe that their kids will be favored in admissions may increase donations.
Of course, if a college (particularly a public one) has a mission to offer opportunities for more disadvantaged students, preferring legacy applicants goes against that. The same goes for a college that wants to be as close to pure merit in admissions as possible; since legacy status is essentially unearned by the applicant, so giving it an additional boost (beyond whatever advantages in achieving merit that an applicant gets from being born into a family with college educated parents) goes against that.