The kunai just seems very pointless. With the power of characters in the Naruto and Boruto series people can easily stop them. They barely ever do damage to people and there are so many different fighting techniques/abilities to use during a fight. The kunai just seems very pointless. Why do the characters use the kunai in battle?
Why bring a knife to a gun fight? Well, it’s better to have it and not need it than to need it and not have it.
Kunais are one of the many tools shinobi have at their disposal. While it may seem like a useless weapon at first glance, there have been instances in the series in which the use of a kunai have been proven quite useful.
If I remember correctly, Kunais are used a lot during close range combat. While up close, it would be easier to thrust a kunai than to attempt to perform a justu.
During the Kakashi vs Zabuza fight on the bridge, Kakashi used kunais to stab and incapacitate Zabuza’s arms to prevent him from performing hand seals.
Kunais have also been shown to used as long range weapons.
A common tactic is to attach a paper bomb and throw it into a crown of enemies. You can hinder with enemy without the cost of chakra.
While performing stealth missions, it’s better to take out enemies with a small dagger than a giant fireball.
Minato was also known for throwing kunais that had his seal to allow him to perform his teleportation justu.
Kunai were historical tools/weapons used by ninja in Japan, though their uses have been expanded/fictionalized in Naruto.
I think in Naruto, they function as versatile long-distance and close-range weapons that can be used even if someone is low on chakra or trying not to attract attention. One thing to also keep in mind is that Naruto and Sasuke (and the like) are the kind of absurd extremes in the Naruto universe. The show focuses mostly on the really powerful people because they make for entertaining TV, but a lot of the shinobi are a lot les powerful and a lot less inclined towards crazy displays, and so good basic weapons are a necessity.
In the series, kunai are also thrown with explosive tags to them (creating projectile explosives), used to cut through wood or stone when powered with chakra, and even used by Minato for his hiraishin.