[jfim]

This would be a pretty long topic to discuss, but in general monetary policy is something that governments really do want to control. If a national economy is not doing well or there is little demand for their goods or currency, the value of their currency is lower, making their exports cheaper and allowing them to participate in the global markets.

One example of having monetary policy out of a country's control is the EU, which has both strong economies (eg. Germany) and weaker ones (eg. Greece). A strong Euro works well for Germany, since they get more effective purchasing power out of their trade surplus. For Greece, a strong Euro means that their exports are probably not competitive price wise.

If these countries had their own currency, the Deutsche Mark would be a strong currency (making their exports relatively more expensive) and the Greek Drachma a weaker currency (making their exports relatively less expensive), giving them a push towards a more even trade balance. As citizens tend to transact in local currency, this also encourages Greek citizens to seek more local goods (eg. Greek tomatoes grown on Greek soil with Greek labor) as foreign goods are more expensive, hence further stimulating their economy.