The form of currency follows its function, which is to serve as a readily accepted medium of exchange of value. Normally, this function rests on a state as guarantor of the value: either as trustworthy guarantor of the kind and amount of metal in a coin, or as powerful guarantor of the continuing acceptance of token coins.
Traditionally, most states have been monarchies where the person of the monarch and the state were equivalent for most purposes. For this reason, the obverse side of a modern piece of currency is the one that evokes that reaction by invoking the strength of the state, and that side almost always depicts a symbol of the state, whether it be the monarch or otherwise.
If not provided for on the obverse, the reverse side usually contains information relating to a coin's role as medium of exchange (such as the value of the coin). Additional space typically reflects the issuing country's culture or government, or evokes some aspect of the state's territory.
Read more about this topic: Obverse And Reverse