The description makes it pretty clear what Ketataumo is used for. Other than immobilizing the target, the constant pain of the technique works to not only damage the victim, but break their concentration, making it a difficult genjutsu to break free from.
*Ace faces his opponent and flows chakra into his eyes, activating his Sharingan. Ace then begins forming fake handseals with his hands elevated to his neck. As he does, Ace channels chakra from his Sharingan and into his finger. As he forms the fake seals, the opponent looks at Ace's hands to read them and get an idea of what jutsu he may use. Using his Sharingan to track the subtle movements of the opponent directing his eyes at his hands, Ace times the point just right, pointing his index finger at the clone right as it looks at his hands. At this moment, Ace flows chakra from his finger into the clone, instilling the Ephemeral genjutsu on him.*