Japan used curved swords, such as the katana and the tachi, for several reasons:

1. Effective Cutting: The curved shape of Japanese swords optimized their cutting performance. The curvature allowed for a longer cutting edge while maintaining a manageable length of the blade. This design facilitated powerful and efficient cuts, as the curvature helped distribute the force along the length of the blade, increasing the slicing power and reducing the resistance encountered during a strike.
2. Draw and Quick Strikes: The curved design of Japanese swords enabled swift and fluid drawing techniques. The curvature allowed the sword to clear the scabbard smoothly and swiftly in a single motion, facilitating rapid deployment in combat. Additionally, the curved shape aided in executing quick strikes and fluid slashing techniques, allowing the wielder to deliver multiple attacks rapidly.
3. Versatility and Maneuverability: The curvature of the Japanese swords provided versatility in combat. The wielder could easily transition between slashing, thrusting, and cutting techniques due to the curved blade's ability to adapt to different angles of attack. The design also enhanced maneuverability, allowing the sword to change directions and follow through with minimal effort, making it effective in various combat situations.
4. Defensive Capabilities: The curved shape of the blade offered some defensive advantages. It allowed for efficient parrying and blocking by deflecting the opponent's strikes along the curve, reducing the chances of the blade getting stuck or deflected off target. The curvature also provided better control and leverage during defensive maneuvers.
5. Cultural and Artistic Significance: The curved swords held cultural and artistic significance in Japan. They were seen as symbols of status, craftsmanship, and the samurai's way of life. The artistry and craftsmanship involved in creating the curved swords were highly regarded, making them not only functional weapons but also objects of aesthetic beauty and cultural identity.

The use of curved swords in Japan was influenced by the country's unique martial traditions, combat techniques, and cultural values. The design of Japanese swords evolved over centuries to meet the specific needs and preferences of the samurai class, resulting in the iconic and highly effective curved blades that are celebrated today.