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With electrical code it is much better to simply follow the instructions and not ask why. There is always a reason, though it is quite possible you might not think of it. I follow some of the discussions that accompany proposed code revisions and even so I can't keep track of all the lines of reasoning that came up on an item discussed a few months ago, let alone years ago. Plus, if you are going to be inspected, you must do what it says, regardless of how much sense it makes.
In this particular case, one of the things they are trying to do is catch any stray currents flowing in the ground in the area where someone could stand and still have their hand in the water. The average distance from the water of someone in that position is about 18" out from the pool. The wire needs to be buried to have the best chances of catching stray currents.