Let's suppose you live in German, all visitors from German can see prices for your products in EUR. It's good for visitors from France, Austria, Spain, Italy as well. But what about UK, Switzerland, Denmark and many other countries which still use their local currencies? I guess it would be good if these visitors can see prices in their currency. UK - £, Swiss - CHF, Denmark - DKK, USA - $. It's quite easy to implement the feature by using this plugin to change your site's currency. Just set up rules for countries and enjoy it!