As far as I can tell China is trying to achieve 2 things:
1) Control anti-Japanese sentiment among its population;

The official party line supports the sentiment of the people in order to appease them. We have seen rioting and other vandalism of Japanese-owned property that would undermine both the CCP's control as well as Sino-Japan international ties (read: trade).

2) Flex muscle in the global political arena.

With China's rise on the world economic stage it now has an opportunity to use a bit of political bargaining power. China's insistence on Japan's compliance with regards to historical events, and Japan's stubborness to submit is all part of the battle of political wills for dominance of the region.