Just another user wrote:
Didn't you read the book? They explain that. Even without the ring Sauron was already winning, with the ring he would have been invincible, but even without it, as long as it exist even if in the deep of the ocean, Sauron could never have been fully destroyed, even if they defeated him, which at that point was highly improbable, he just would have reformed somewhere else as strong as before, so the only thing to do to get rid of him was to destroy the ring.
Yes, they gave a reason and it sucked.
Honestly I don't even know why people cared what Sauron did. He was a giant floating eye that couldn't really hurt you except if you carried the ring which let him try to corrupt you. It wasn't so important that you destroyed the ring, so much as that you prevented him from getting it.
Sauron wasn't winning the war, the orcs were. You could seriously kill off Sauron and if the orcs kept attacking, they'd have won. And of course, I'm not even certain the orcs were even winning, given that they destroyed Isenguard and Gondor fought off the invasion, pushing them back into Moria.
Seriously, without destroying the ring, they managed to basically fight off the entirety of Sauron's forces and push them back into Mordor, continuing the standoff that Gondor had for centuries before that.
The plan of sending Frodo (or really any small group) into Mordor with the ring was a pretty foolish one. You allow for the possibility that the ring falls into enemy hands (it nearly did quite a few times), and then you're really screwed.
You're better off just learning to live with the giant eyeball and dropping the ring to the bottom of the ocean. Seriously if the Nazgul want to spend thousands of years looking for it, let them. They barely found it when Frodo had it, and that required lighting signal fires on Weathertop and shit. These are guys who literally can't sense the ring when it's under their noses, so long as someone isn't wearing it.
I find it really hard to believe that 9 guys searching the entire ocean would have any realistic chance of turning up anything. The ocean is fucking huge. Modern search teams have difficulty finding sunken ships in the ocean, and that's at shallow depth. Finding something as small as a ring would be a fucking miracle.