The Universal Tenkara Rod Cap size small was made to fit the Rhodo, Sato, and Ito rods. It goes over the top end of the rod, offering better overall protection to the rod's end. It is also easier to keep track of, and if one chooses it can even be wrapped around the rod itself while fishing. Lastly, the line may remain attached to the rod while it is used.
This product is being offered as a result of people saying they had a hard time keeping track of their little plugs, and also as a spare piece that will fit any tenkara rod. The plug that comes with your tenkara rod is perfectly fine if these things do not bother you.