Katanas were traditionally folded during the forging process for several reasons:

1. Refining the Steel: Folding helps to refine the steel and improve its overall quality. Each fold allows the swordsmith to remove impurities, distribute carbon more evenly, and create a more homogenous steel composition.
2. Enhancing Structural Integrity: Folding and layering the steel creates a laminated or layered structure within the blade. This structure provides strength, durability, and resistance to deformation during combat. The layered construction helps to prevent the blade from breaking or shattering under stress.
3. Developing Distinctive Patterns: Folding and layering the steel results in the formation of distinct grain patterns known as "hada." These patterns are highly prized for their aesthetic appeal and uniqueness, adding to the visual allure and artistic value of the katana.
4. Eliminating Welding Imperfections: During the forging process, different pieces of steel are joined through forge welding to form the blade. Folding helps to ensure a strong and secure bond between the layers, removing any potential imperfections or weak points in the welds.
5. Achieving Consistency: Folding contributes to achieving a consistent distribution of carbon content throughout the blade. This allows for a uniform hardness and edge retention, which are essential for the katana's cutting ability.

It's important to note that while folding was a traditional practice in Japanese swordmaking, modern manufacturing techniques and advances in steel production have made it possible to create high-quality katanas without extensive folding. Some modern swordsmiths may choose to incorporate folding techniques as a nod to tradition or for artistic purposes, even if the practical benefits are minimal.

Overall, the folding process was a key aspect of the traditional Japanese swordmaking craft, contributing to the strength, durability, and aesthetic beauty of the katana.