No satisfactory explanation has ever been provided by an official source as to why central banks hold gold, despite Ron Paul badgering them. The most he could squeeze out of helicopter Ben was 'tradition'.
The real answer I suspect is that they recognize a reasonable probability that their house of cards---known as fiat currency---has a short historical shelf life. Thus they want to make sure they have hedged their bets in a very literal sense. They know that gold is the most likely Schelling point if their reserve currency game falls apart, so they want to make sure they'll still be on top if that happens. Remonetized gold would probably be way more than 10k in current dollars per ounce.
There is also, I suspect another aspect to them holding gold. By owning a large portion of it, especially a large portion of the portion of the total gold that actually circulates somewhat (an awful lot of the world's gold is tied up in jewelry that only liquidates under very extreme circumstances, e.g. that purchased by fathers in developing nations), they have profound ability to manipulate that market. Since the price of gold is a pretty good proxy for how little the population trusts the banksters and their governmental minions, being able to fox that metric is pretty damned useful. A very steeply rising price of gold, I'd wager, makes its remonetization (and the corresponding collapse of the old financial order) more likely. I seem to vaguely recall Greenspan saying something similar to this back in the days before his reign at the Federal Reserve.