Why does sauron need the one ring?

Answer 1

Sauron needs the one ring because it is his life. He and the ring are bound to one another. Sauron really did die, but since the ring was not destroyed his spirit remained in Middle Earth. If the ring was destroyed, then Sauron would finally die.

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It's really a very simple answer:

Sauron is made up of a life force, which before the ring was made, put him at 100% life force. He then decided "Hey, I can put some of this life force into a ring, and then twist it so that I have awesome super powers." So he did. Before you know it, he's going house on people and teaching them what's what.

Then Isildur grabbed up the sword of his fallen father, Narsil, and was like "Not on my watch" and wrecked him, taking the ring for himself. Sauron was basically like "oh crap" because like 99% of his life essence just got taken away from him and he basically only has enough to command his troops and manifest as a fiery eye. Even that, however, is tenuous, for it is only doable because out there, somewhere, the rest of his life essence lives on in the form of this super-powered ring.

So, if that ring is destroyed, he loses the ability to manifest at all. Thus being destroyed.

Answer 2

Sauron did not "have" to forge the One Ring of Power. It was rather a gambit, a strategy he chose to employ to defeat his enemies. Sauron did not believe himself powerful enough to take on the powers of Middle Earth - that is the elven, human, and dwarvish kingdoms - so instead he sought to corrupt their leadership. Sauron, disguising himself in a pleasant form and pretending to be a benign entity spreading knowledge, taught elven smiths to forge the lesser rings of power and these were then freely gifted to the great kings of Middle Earth. However, the power of these rings was somehow designed such that they could be subordinated to the ruling One Ring which he forged in secret. Through the One Ring Sauron would be able to perceive, corrupt, and ultimately control any who wore the lesser rings. Unfortunately for Sauron, the only way he could make the One Ring powerful enough to rule over the lesser rings and those who wore them was to pour some of his own spirit into the ring.

Ultimately, the scheme met with mixed results. At the moment the One Ring was forged and Sauron put it on, the elven ringbearers are reported to have understood his true nature and removed their rings. Dwarves and men appear to be less perceptive and continued using their rings, presumably becoming addicted to the power the rings granted them and corrupted by Sauron's influence upon them. The dwarven ringbearers appear to have used their rings to pile up immense wealth, but this wealth ultimately became their downfall as it made them targets for dragons and similar enemies. The human ringbearers were completely consumed by the rings and became the Nazgul.

Thus, Sauron created the rings in order to corrupt and subordinate his most powerful enemies. And while the plan was successful in many respects, the necessity of infusing the One Ring with much of his own power did create an Achilles heel that ultimately led to Sauron's own destruction.