In anarchist literature, one often finds the contention that the State produces nothing, and is entirely parasitic on the rest of society.
This claim is false. Of course, it is true for some things that modern states do, such as the provision of welfare. But it is false applied to the state as a whole, because there is one service that is highly productive, and that only the state can provide: the service of being the final arbiter for all disputes between its members. This service must, logically, come from a monopoly provider: if there are multiple providers of arbitration at the same level, then none of them are final. (And that is why, if a network of ancap defense agencies can provide this service, they will, in fact, compose a state. And if they can't provide it, then we will have "anarchy" in the bad sense of social chaos.)
Once one focuses on this service, one can easily understand why German barbarians would fight to get inside the Roman Empire: both productivity and security go up in the presence of such a final arbiter. And that is why we have never seen any wealthy, stateless societies.