The document discusses the importance of grades for motivating students. It argues that without grades, students would not work hard and instead enjoy their youth without effort. Grades provide a scale for students to judge their own performance and positions in class. This motivates students to work harder if they want higher positions and to achieve their goals. Similar to farmers working hard for good crops and money, students work hard for good grades that are important for their future careers. Grades reward good work and motivate further effort.