And the useful purpose is why currencies are worth something.
No kidding, but the whole discussion here is about what that worth will be in the future. Is anyone saying the US dollar has no worth today? Is anyone saying bitcoin has no worth today?
A person going to the market in Venezuelan to buy chicken doesn’t care about the bolivar’s forex exchange rate against other currencies. They care that the government devalued the bolivar another 20% that week, and the bolivars in their possession LOST VALUE by that amount when buying chicken.