Feedback to the student about their level of mastery of the coursework (relative to the coursework itself, not the rest of the class). Of course, that doesn’t

Feedback to the student about their level of mastery of the coursework (relative to the coursework itself, not the rest of the class). Of course, that doesn’t have to be a letter; percentages along with specific corrections would do as well. But once there’s a convention people often use it. Grading students relative to each other gives different information and probably not precise across cohorts at that.