Because war is as much about money as it is about bullets, bombs and missiles - financial health is an important quality to consider.
Reserves of Foreign Exchange and Gold allow a nation to repay its foreign obligations (debt) while also continue to strengthen their domestic currency. These reserves also play a part in a nation's credit rating on the world markets. Today, it is said that as much as two-thirds of the world's reserves are held in Asia by China, Japan, Taiwan, Hong Kong and South Korea. For the purposes of the GFP ranking, this plays into the relative financial "health" of nations.
The data presented below is through 2017. Values presented in $USD.