It is not for financial aid!
AMCAS starts collecting information that it then combines with admission and matriculation data, board scores, 4 and 5 year graduation data, etc to determine predictors of success in medical school. One concern is that students from low income families do not do as well (less likely to graduate in 4 years, or 5) and to be able to test that hypothesis and, if true, to provide additional supportive services to students who may be in need. It also makes it possible to control for family income when examining graduation rates, etc by race given that what is sometimes identified as differences by race are perhaps more correctly identified as differences by family income.