They don't need to join the EU to use the euro. There are countries that use the U.S. dollar even though they're not the U.S. Sometimes it's just de facto use the dollar. There is technically a local currency and people use it when forced to but they would prefer the dollar. If you're a small country, something like the dollar or the euro is probably your best bet because of the stability it affords. The advantage to joining the EU is Iceland gets a little bit of input on how the euro was run versus if you use the dollar of the euro when you're not a member of their respective country you are partially tying your financial stability to something that you have no input or control over.