That relies on a third party to maintain the value of the currency. Maybe I'd prefer to minimise my dependence on third parties. The crypto ecosystem makes it much easier to trade six cows for two tons of lumber and offload all of the exchange risk to speculators. It still can't enforce the physical exchange of goods, but it can atomically negotiate a swap from anything to anything through as many intermediaries as required. With a suitable transaction fee — which may come out of the transaction inputs.

A cryptocurrency system can negotiate that my six cow NFTs (which are not jpegs, I guarantee they are exchangeable for cows) for two Ethereum, of which 0.05 is taken as a fee and 1.95 flows on to Uniswap to buy Home Depot coin which buys two tonnes of lumber at a specific location and time. Except oh dear — Home Depot only trades on Polygon. Well then my 1.95 ether is bridged to Polygon where my 1.94 wrapped ether is traded for 9999999 POL and then for Home Depot coins. Or I buy wrapped Home Depot coins atomically on Ethereum and then unwrap them back to Polygon.

Crucially it all happens automatically.

That infra doesn't exist since cows and lumber aren't being traded on a blockchain, but it's the kind of thing that could be enabled.

Since I only hold ETH, WETH and POL during the transaction, their absolute value doesn't matter.