Because gold can't be printed ad infinitum like the central banks can and do with currency. E.g If you hold Japanese yens and the Bank of Japan announces it is

Because gold can't be printed ad infinitum like the central banks can and do with currency. E.g If you hold Japanese yens and the Bank of Japan announces it is printing like crazy like they did recently, and the currency falls 20% in one year, you loose 20% of your reserves. Useful for what? For Bankruptcy? Today all central banks are printing like crazy, so it is a good idea to hold real things.