Tinnitus is a constant whine. Other actual noise helps prevent having to constantly be thinking and feeling the whine because the "white noise" has your ear dealing with other things.
It isn't the reason everyone listens to white noise but people with Tinnitus could help themselves go less crazy if they listen to white noise.
It was recently discovered that the reason tinnitus is a thing is not psychological at all, it's a degeneration/damage of the nerve endings in the ear. So since they found that, don't give up hope that they'll find a treatment for it.