“What is the practical benefit of this machine??? “
None. Besides, if anyone is drinking so much soda that they need a cost benefit to buy the thing, maybe they should just cut back on it.
My reason is the complete opposite. We drink so little soda that if we buy a two liter, at least half gets flat and thrown out. We make a half liter bottle and waste none, in two years, I’ve bought one carbonator bottle. It also is good for carbonating water for mojitos!