Why do some US universities request parent's educational level in graduate admissions application?
Many institutions have an interest or incentive in serving first-generation college students, i.e. students whose parents do not have college degrees. This can be seen as a way to reduce educational or economic inequality.
It's possible that in some cases there may be some sort of preference given to such students. More likely, it's for statistical purposes only, to see how well they are doing at reaching first-generation students.