Something I've never quite understood... (Why a Ring?)

[Voralphion]

Furthermore, why tie your life-force to something physical at all? Why not ferret it away on the astral plane or somewhere else safe?

Sauron had to put his power into the ring so he could control the others. His ring had to be more powerful than the others so only his direct physical power would be enough in order to be more powerful than the elven rings