Over thousands of years, gold coins have been made to many different standards of purity. By the 19th century, most countries struck gold at either .9166 (22 carat) or .900 fine, depending on whether their coinage systems were influenced more by British (22 carat), or continental European standards (.900). For example, the USA used to use the continental .900 fineness in its gold coins, whereas Canada used 22 carat.
Twenty-two carat has been found over millennia to provide a good alloy for gold coins. It is almost impervious to chemical attack and tarnishing, yet hard enough to withstand decades of wear and tear. Nine hundred gold is very close to 22 carat and shares its properties.
It is also a popular misconception that more the base metal, the harder the alloy. The fact is that alloying creates controllable properties, so that alloys can be softened by annealing and hardened by working or quenching.
The original purpose of gold coins was to circulate as everyday money so, they needed to be durable. Coins intended purely for storage as bullion coins do not have this requirement and there is no reason not to use pure gold.
However, in practice, it is difficult to refine any substance to 100% purity. 24 carat gold is a slight approximation, it is more accurate to refer to it as .9999 gold.
There is a possible advantage that fine gold does not need to be alloyed, therefore removing several production stages. Hence, pure gold coins can be produced more cheaply than alloyed ones.
While British Britannias, American Eagles, and South African Krugerrands are all made of 22 carat gold.
Austrian Philharmonikers, Australian Nuggets, Canadian Maples, Chinese Pandas and Indian Gold coins are all made of 24 carat gold.