Can I ask what's the purpose of a global savings account for different currencies?

Meant to diversify currency risk. however should only include "stable" currencies,

if meant for saving.

Currency conversions can be done in SG by Your bank, but this is way to expensive.

If you are in need to convert currencies, use cheaper(and safe) Services

like Transferwise or CurrencyFair. If you f.ex. have in SG two bank accounts

in nominal SGD and USD currency, You can do the conversion even within Singapore.