Agree with all that is said about taking on a physical body to interact with the physical world. In most cases, the Ainur retained tha ability to abandon their earthly body and revert to their natural form. My understanding however is that, the more an Ainu indulged in taking on a physical form, the greater the likelihood that it would become permanent. This, I assume, is what happened to Sauron.
As for the One Ring, while its main purpose was to gain control (via the other Rings of Power) over the Free Peoples of Middle-earth (and particularly the Elves), it also, as Boro has indicated, made Sauron practically invulnerable. Having taken on a physical body, that body could not permanently be destroyed while the One Ring remained in existence (or, if it was, he was able subsequently to rehouse his spirit in another). And given that virtually no one could willingly destroy the One Ring or master it to the exclusion of Sauron, it made him more or less invincible.
But for two brave Hobbits and a wizened proto-Hobbit with dual-personality disorder ...
