Are you talking about why we would want to do the spiraling or why we would want to do shiko?
Aren't they the same thing? There's a particular reason to do Shiko, as opposed to say Fune Kogi Undo, but for all practical purposes they're just facets of the same jewel. But I'd be interested in either hearing discussion about why Shiko particularly as an exercise or why winding in Shiko. Maybe Dan can chip in, since at the beginning of the post he was telling Chris:
Whatever you do ignore everything in that Sumo link Chris provided. It's nothing a martial artist wants to do. This is NOT the Shiko you have read us talking about nor want you want to be doing.
What exactly is a martial artist doing in Shiko that is so helpful? Why is winding/spiralling important? And so on. The discussions about the how and why would probably be helpful to everyone.