Why did Sauron need ring?

Sauron was certain that the Ring would be used against him. By the time he learned of the whereabouts of the Ring, much less after factoring in the time it would take to march his forces north across hundreds of leagues, he could reasonably expect that the Ring would have been moved. In fact, the Ring did leave Rivendell after only two months, which is probably faster than Sauron could have heard of the events and sent an army north.

As we learn in Unfinished Tales Isildur, with a small force of hardy Numenoreans, expected it would take him 40 days one way. Sauron would need longer than that even to just march his army and gather his forces.
Sauron would only know that the One Ring had been delivered to Rivendell and he would not know this right away. At Rivendell were several very powerful Individuals that, with the One Ring in (or on) hand could defeat Sauron. Add to this the idea that Tolkien himself gives as Sauron's greatest error in judgment, which was a belief that a person of power who gained control of the ring would act as he would.