The main objectives for the PBOC to accumulate gold are:

   Supporting the renminbi for its internationalization (adding trust and credibility)
   Owning hard currency as the cornerstone of capitalism.
   Owning reserves that protect the Chinese economy from external/internal shocks and inflation.
   Owning reserves that are not controlled by a foreign nation (the US).
   Diversifying its excessively large USD reserves prior to an irrevocable USD devaluation.
   Hedge their exorbitant USD reserves.

In my opinion the PBOC (or its proxies SAFE and CIC) does not purchase gold from domestic mines or from the SGE. The PBOC’s incentive is to exchange USD’s for gold, preferably buying undervalued gold with overvalued dollars. Hence the PBOC buys in utmost secrecy, not to affect the market.