1. Balancing the Character: Kakashi Hatake's weakness in chakra is essential in preventing his character from becoming too overpowering. With his Sharingan, he acquired the nickname "Copy Ninja Kakashi" and mastered thousands of jutsus. However, if he didn't suffer from chakra depletion, his abilities would overshadow even the Third Hokage.
2. Allowing Protagonists to Shine: Kakashi's chakra limitation allows other characters like Naruto, Sasuke, and Sakura to take the spotlight as protagonists. While Kakashi initially stole the show, his weakness allowed for the growth of the main characters and shifted the focus away from him.
3. Enhancing the Story's Progression: Kakashi's immense talent and eventual position as the Sixth Hokage demonstrate his power surpassing even Itachi Uchiha. However, without his chakra weakness, he would become too powerful, leading to a different outcome in the Fourth Great Ninja War and rendering Naruto and Sasuke's journeys irrelevant.
In conclusion, Kakashi's weakness serves the purpose of balancing his character, highlighting other protagonists, and enhancing the overall progression of the story.