The combine is great at providing tiebreakers for prospects with similar film grades. And remember, college football players do not play against equal levels of competition. Viewing their athleticism (as well as interviews, medicals and other tests to which we do not have access) helps teams sort out similarly talented prospects who might not have many common opponents. Players might also be asked to perform different tasks on an NFL field (within different schemes) than they did in college, so comparing their relative movement skills helps sort them into different tiers.