There are also many countries without their own currency. For example, Ecuador and Panama both use the USD dollar as their official currency. See this list, which includes
There are several reasons why you would like to adopt another country's currency, but common factors are
high inflation: the domestic currency loses value against foreign ones. Therefore, people start more and more to use foreign currency for transactions (e.g. dollars in Venezuela today). Therefore, one solution is simply to get rid of your own currency and adopt a foreign one. This immediately stabilises prices, as the central bank cannot longer print money.
high levels of trade with one country: for example, if you trade very much with the US (as many small Caribbean islands do), you would rather adopt the USD dollar. This will give much more certainty to exporters and business in general.