Because they are going to debase the Dollar as the world reserve.
"Debase" or "maintain confidence", take your pick. Actually, nobody knows for sure what's going on there, but what is for sure is that lots of pundits are coming up w/ possibilities (from Forbes):
The People’s Bank of China, for the first time in more than six years, revealed a change in the country’s official gold reserves. On Friday, it announced it held 53.32 million troy ounces of gold as of the end of June. That’s up 57.3% from 33.89 million ounces at the end of April 2009.
China, not Russia, now has the world’s fifth-largest holdings of the yellow metal, but analysts are not impressed. Some, like Brien Lundin of Gold Newsletter, think Beijing’s holdings had about doubled during the period. Some believe they had jumped three-fold. Others say more, with China possessing as much as 3,510 metric tons. If the high-end estimates are correct, China could be hiding somewhere in the vicinity of 1,850 tons.
The assumption that China still has not disclosed all its gold has led Lundin, like many others, to wonder “why they would feel compelled to understate the total now.”
The thinking of Chinese officials about their stockpile remains a mystery, but there are various theories. The most benign comes from GoldCore’s Mark O’Byrne, who said China may have been “lowballing” the size of its holdings to maintain confidence in the dollar.
Are the Chicoms lying about how much they have so they can one day soon lift the veil and destroy the dollar?
Now the Chinese can buy more cheap gold.