The settings control the default marketplace currency, this currency will be used to format areas that display money or financials such as site fees. This will also be the currency that all merchant gateway payments for buyer orders will be settled in unless the currency selector is on and a different currency is selected for that specific item.
Currency manager lets you add and manage all types of currencies for your marketplace. If your marketplace supports only USD you can safely remove others.
All master currency rates are updated every 15 minutes through the automation system.