The answer is simple: fiat paper money that is not grounded in any objective standard can be manipulated at the whim of the issuer. Without the requirement to exchange fiat money for gold or some other commodity, the central bank can issue unlimited amounts, thus making its value subject to extreme volatility and, as we have seen, perpetual debasement.
http://www.zerohedge.com/news/41-years-after-death-gold-standard-look-how-we-ended-economic-purgatory
