Can you be more specific about the countries to which you're referring?
I know that Timor Leste uses the US dollar as it's currency. If you don't pay your taxes to them in US dollars, you go to jail.
If you're talking about black market transactions, they hurt the local economy.
People living in any number of third-world countries whose national currency is shitty and prone to exorbitant inflation rely on the US dollar instead.
Are you talking about black market transactions?
Not really, you can buy US dollars in official exchange houses virtually all over the world.
Okay, but those dollars aren't bought for countries to use in their own private sector - they use them for importing goods (that's the foreign sector).