"The Treasury's sales of U.S. debt to China have become part of a politically charged public debate about China's role as the largest exporter to the United States and also the country's largest creditor.
The privilege may help China obtain U.S. debt for a better price by keeping Wall Street's knowledge of its orders to a minimum.
Primary dealers are not allowed to charge customers money to bid on their behalf at Treasury auctions, so China isn't saving money by cutting out commission fees.
Instead, China is preserving the value of specific information about its bidding habits. By bidding directly, China prevents Wall Street banks from trying to exploit its huge presence in a given auction by driving up the price."