It allows to to copy your partners movements; Sharingan's trash; The user is granted incredible clarity of perception, enabling them to read lips or mimic something like pencil movements; It's great in trapping someone in a genjutsu; The Eternal one gives you a cool armour; In combat, this allows them to see fast-moving objects and, once fully developed, offers some amount of predictive capabilities. The Sharingan eventually develops into the rinnegan