On Currency Declarations

In Angola its not allowed to take local currency out out the country. I not sure why anyone would want to  - much like Yemeni riyals and Bangladeshi taka you will have a tough time finding a moneychanger  willing to exchange your kwanzas (buy or sell) for a more serious currency. The law mainfests itself with a currency declaration (and accompanying queue) each and every time you leave the country.
I'm curious to know the reasons why. Are these declarations designed to prevent large outflows of local currency? How would it benefit someone to carry local currency outside the country if they were unable to exchange it? Is it merely an artifact from an earlier time? Anyone know the reasoning?