It was simple enough to me. The Ring "belonged" to Ryou, and no matter what, we could never get rid of it. So I had originally started looking for a way to free him of you; giving you your own body, or the ability to manifest one, was the most obvious and acceptable way to do so. In the latter case, that would even spare everyone else from you.

You are hardly one I would hold close to my heart, but in a way, I like you, Spirit. Perhaps I see too much of myself in you and unwisely lean on that, but I do. So it ended up being a way to spare you as well.