It could give them an indication of why you went to the college that you did for undergrad. Some people choose to go to lower-ranked schools because of a great scholarship or specialized program, while others choose their school because they can't get into a better one.
This knowledge could help them evaluate the legitimacy of a great GPA from a third-tier school. If you had low scores and grades in high school but come out with a great GPA from a low-ranked school, they might assume that your high GPA is due to a lack of competition from your peers.
Your MCAT score will carry more weight along these lines (showing how legitimate your GPA is), but your high school stats can shed some light as well.