Most of the other comments have explained it, but I'll also point out that in many developing countries or ones that have recent crashes in their economy, their

Most of the other comments have explained it, but I'll also point out that in many developing countries or ones that have recent crashes in their economy, their unofficial-official currency is the US Dollar or, less commonly, the Euro. USD and EUR are often stand-ins for the country's currency for various reasons. This leads to devaluing of the currency and eventually laws on money exchanging and import of other currency (see also: China).