[Raynor]

But why would a Maia want a body? Why would a spiritual being want to confine his power to a Ring, or any other material object?

Cf Silmarillion, Sauron "walked behind [Melkor] on the same ruinous path down into the Void"; Sauron must have followed Melkor's modus operandi:

Originally Posted by Notes on motives in the Silmarillion, Myths Transformed, HoME X

Melkor 'incarnated' himself (as Morgoth) permanently. He did this so as to control the hroa, the 'flesh' or physical matter of Arda. He attempted to identify himself with it. A vaster and more perilous, procedure, though of similar sort to the operations of Sauron with the Rings... But in this way Morgoth lost (or exchanged, or transmuted) the greater part of his original 'angelic' powers, of mind and spirit, while gaining a terrible grip upon the physical world.

I've heard (don't necessarly believe it myself) that demons and other spirits and the like who sometimes posses people enter into a physical body so that they can interact and touch (physically and through the mind with words and feelings and such) other people and/or things. So that, more easily, they can influence what happens.

Sauron was called the Necromancer, and he learned his skill from Melkor himself; we also have the evil elvish souls, who, after death, could attempt to inhabit other bodies, but not through the lawful ways, set by Manwe, as told in Laws and customs of the Eldar, HoME X.

Perhaps having something through which to channel his power made it easier to use said power.

From the begining, the purpose of the one ring was to 'govern' the elven rings, cf Of the rings of power, Silmarillion. Tolkien doesn't give a measure of how much his power increased through the one ring, only that it was "enhanced"; however, we are told, through Gandalf, in the Last Debate, that should Sauron recover it, his victory would be so complete "that none can foresee the end of it while this world lasts", something I doubt would have happened without the ring.