The other is that existence is truth. This is the same in any country.

Like Bolivia, for example, they don't allow foreign exchange to flow out now, and there is so much international trade. What if they don't take the black market?

Prior to this, enterprises in China were not allowed to keep foreign currency, and they had to apply for traveling abroad to exchange money. All kinds of inconveniences have caused the existence of the black market.

Another and most important point is money laundering.