The Straight Road

[Ibrîniðilpathânezel]

Originally Posted by Eomer of the Rohirrim

On that note, why bother with the curve? If the blessed realm was to become another plane or dimension, whatever you want to call it, why not just make it so where it stood? Surely it wouldn't matter to everyone else where it stood in their plane. They couldn't reach it anyway.

Good question, and I surely don't know why. It's just the way Tolkien describes it. Perhaps he intended them to teleport to another planet rather than achieve interdimensional transit, and that's what happens at the moment the ship vanishes. He never does say. But what he does indicate it that they are leaving the world of Arda, for good, and going to another. There is no way back, except for the Ainur, because they can move disembodied. For the Istari to make the trip in real bodies represented one of the "exceptions" that Tolkien does admit pop up in these kinds of stories, and that particular exception was done with the approval of Eru. Having the ship leave the curvature of the Earth might have been a way of making the physical departure from Arda a real moment of detachment.