It's a lesson on the benefits of the securitization of debt. If people could have traded their debt, they could have zeroed out the whole thing without the Germans.

Honestly, there isn't that much benefit from this story. Everyone in the story went from a net $0 net worth to a net $0 net worth. I'm sure clearing debts (or credits, depending on your perspective), from the books does benefit people, particularly psychologically, but the idea that everyone became massively better off is pretty silly.