To give shape to their nothingness in their dealings with the living.
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maybe the answer is that the Nazgul were incapable of, for example, picking him to carry off to Mordor unless they were doing it with something material like a glove or long sleeve.
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Yes, that's pretty much it. I think that without a visible form they might have had difficulty in physically doing anything, like grabbing Frodo or taking the Ring.
They were not invisible in the same way as Frodo or Bilbo when they wore the Ring. The Nazgul had actually fallen into a state of semi-existance that rendered normal interactions impossible. Thus, they had a need for visible forms so they could relate to the world around them.
And, they did not seem so very easy to avoid. If it had not been for the intervention of Strider, they would have gotten the hobbits. They almost did anyway.