What is newproxy and how is it useful?

I was messing around with Lua yesterday and stumbled upon the 'newproxy' function.

http://wiki.roblox.com/index.php?title=Function_dump/Basic_functions#newproxy

I kind of understand it, but I'm not sure how it is useful. I know it creates a blank userdata object with a metatable attached (if the argument is true).

How it newproxy useful? Here is an example of what I did while messing around with it:

local proxy = newproxy(true)
local metatable = getmetatable(proxy)

metatable.__index = function(array, key) print(array, key) end

local y = proxy[100]

--[[
    OUTPUT:
        userdata: 0x443ad4b4 100
]]