It is very usefull actually.

Imagine if you can control your opponents body through their sword, that is to say you can disrupt their kamae, take their balance etc via sword on sword contact. This enables you not only to manipulate their center but have control of the center line. Likewise you can cut "through" your opponents sword enabling you to knock away their sword (potentially their balance as well) and keep control of the center line if you both cut at the same time without cutting any "harder."

What if you are using kokyu to derive your cutting power? What if your cut is the same as your block?