What is the point of using the kunai? 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. 

Close Range
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. 

Long Range
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.