Right now we have classic, 1GP=10SP=100CP,
but prices of those metals are not in that range,
if we go to 1GP=100SP=10,000CP we would come pretty close to real prices for those metals.
also it would make CP and SP meaningful in campaigns, right now SP is maybe used at 1st level, and CP in next to unheard of.
also it would make transporting large amount of wealth more easier.
Fullplate costing 15GP is more manageable transaction than 1500GP, carrying around 30lb of gold is a hassle. 15 coins still can fin inside a pocket.
I can hardly imagine a blacksmith having chests full of gold after a busy day
then platinum can be made as some magic crafted metal that can hold magical essence for enchanting and have 1PP=100GP.
Platinum would only be used as magical currency or royal exchange.
common person would never see a platinum coin in a life probably, and only occasionally see a gold coin.