Japanese swords, including 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. During each fold, impurities and other unwanted elements are removed or distributed more evenly throughout the material. This process helps to create a purer and more homogenous steel composition.
2. Homogenizing Carbon Content: By folding the steel, the swordsmith can distribute the carbon content more evenly throughout the blade. This helps to achieve a consistent hardness and allows the blade to hold a sharp edge.
3. Enhancing Blade Structure: Folding contributes to the creation of a layered or laminated structure in the blade. The layers provide strength and flexibility to the sword, allowing it to withstand impact and bending without easily breaking.
4. Creating Distinctive Patterns: Folding and layering the steel results in the distinctive grain patterns known as "hada" in Japanese swords. These patterns contribute to the aesthetic beauty and uniqueness of the blade.
5. Removing Forge Welding Imperfections: During the forging process, the swordsmith joins different pieces of steel together through forge welding. Folding helps to ensure a strong and secure bond between the layers, removing any potential imperfections or weak points in the welds.

It's important to note that while folding was a traditional practice in Japanese swordmaking, modern advancements in steel production techniques and heat treatment have made it possible to create high-quality swords without extensive folding. Some modern swordsmiths choose to fold their blades for aesthetic purposes or to pay homage to traditional methods, even if the practical benefits are minimal.

Ultimately, the decision to fold a sword blade depends on the preferences and techniques of the swordsmith, as well as the specific requirements and desired aesthetics of the sword being produced.