See my response to your other question here: https://news.ycombinator.com/item?id=27681781 (but like, the idea is that you can spread the cost of an underlying transfer--which costs money to perform and potentially permanently store on the underlying blockchain--across all of the people making probabilistic payments, which you can make very small if the people involved are willing to tolerate the variance on a resulting purchase and have a high enough principal to still have a reasonably high efficiency after paying for the amortized fees).