Money turns out to be the most fungible of these, since it can be (more or less) traded for the others.
Right now, there are a bunch of economies being bootstrapped, and the bots will eventually figure out that they need some kind of fungibility. And it's quite possible that they'll find cryptocurrencies as the path of least resistance.
> Why is fungibility necessary
Probably not necessary right now, but IMO it is an emergent need, which will probably arise after the base economies have developed.