The katana "Ha" is the cutting edge. Due to differential hardening, this portion of the blade will be substantially stiffer and harder than the remainder of the blade. This hardness contributes to cutting ability in an equal or greater degree than the sharpness of the blade.
A sword needs to strike a balance between sharpness and durability in order to function properly. A blade that is too sharp and too thin could easily break, hence the importance of the balance between hardness and sharpness.