A meditator who always sees others but not himself is following a hopeless mental exercise. One must concentrate on one's own heart and mind and penetrate deeper into them. If one only looks superficially, one will not understand one's own mind. Hopeless mental exercise
So the purpose of (useful) mental exercise is to see oneself. To see what? ... To see corporeality (Rupa ), feeling (Vedana ), perception (Sanya ), intentional activities (Sankhara ), and consciousness (Vin