Entering a parents info allows the school to see what kind of background you come from. Every single applicant/accepted student at this stage is relatively poor, therefore, if schools went off of a student's income, everyone would receive need-based aid. By providing your parents information, you provide schools with a measure to gauge poor vs. non-poor.
Note, some schools do not require your parental info for need-based aid if you are married or, for example, if your parents are incarcerated, etc.