Tom Bombadil, as part of his song to battle the wight which captured Frodo and Co., says "Vanish in the sunlight!" so perhaps the Barrow-wights were stuck in their hills, and therefore couldn't be used by Sauron in his war.
The Nazgûl, when telling the Dwarves of Erebor they would be reimbursed for information about the whereabouts of "Baggins," could have been thinking of using the riches in the Downs. Otherwise, I can think of no reason to hording the gold and such. Perhaps it's related to dragons hording riches; they too will never use it, but have an insatiable lust for it.
It's also possible that the Wights stayed in the Downs as guardians of their master's fallen realm, just in case he ever came back.
I have no idea of the purpose of clothing the Hobbits in their finery. The purpose of sacrificing the Hobbits, I don't know. Don't even know who they were being sacrificed to, though that's probably just up to speculation.
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