Money Faces

Could someone please tell me why is it necessary to have a picture of a country's who's who on its currency? One reason i could think of is historical; Ancient Roman coins, from my understanding, have the face of its ruler (at the time of minting).

Anyway, a creative group (i believe they were really bored) of people found a way to prove that money could also be used as a mode of entertainment instead of being a medium for entertainment!

 Interesting yah?

It must be, because someone tried using Singapore currency too!