Unless, of course, it's illegal for normal citizens to own foreign currency.
If foreigners can only own foreign currency, and normal citizens can only own local currency, then it makes it a lot harder to bribe the locals!
If foreigners can only own foreign currency, and normal citizens can only own local currency, then it makes it a lot harder to bribe the locals!
The three-tier model was a little but unusual, but the idea of a secondary currency for foreigners was not that uncommon.
One other reason for it is to ensure "minimum spend" -- some countries required you to buy a certain amount of the foreigner currency at the official exchange rate as part of the entry process, even though the black-market rate for "local" currency was far more competitive.