As title says, certain weapons counter certain weapons.
Spear (Yari) vs Knife (Tanto).
While often longer range wins, there are methods of dodging or parrying the spear's attack & closing the distance - at which knife has advantage.
But if defense is 'overdone' - moves too large, it will fail.
Yari is not useless at closest distance, for it can be used to push opponent away.
See also: Is Ideal Strategy Ideal?, Teamwork.