They already have low yield nukes but to develop high yield, compact warheads without much nuclear testing (which can be easily detected and would undoubtedly draw critics from other countries) precise simulation on super computer is necessary. Also to solve fluid dynamics problems (for example to develop advanced jet engines or silent submarine propeller, which the Chinese desperately need), detailed simulation on super computer is absolutely a must. Now running LinPack to show off some nice PR numbers is one thing but actually writing an own efficient (and correct) simulation) on massive parallel systems is a thing of another level. It surely helps if one can draw on the massive highly optimized numerical libraries built for the x86/64 instruction set. It certainly will not hinder either in case one has stolen properties software and code from western companies!