Alternative, non-state-controlled currencies serve a basic human need under authoritarian regimes.

They also enable trade to continue between nations that trade-sanction/blockade one-another, which is usually seen as a global-economic net good (in the sense that a trade-sanction/blockade is essentially an infinity% tariff, and economists have analyzed to death how tariffs hurt both the target and source economies.)

(Some countries accomplish this by just using some other country’s physical currency — e.g. Venezuela using USD. But in really authoritarian [and technologically-modern] countries, any type of movement of physical currency exchange is heavily tracked, so this isn’t usually an option.)