Why does China occupy Xinjiang?

The inhospitable terrain, especially the Taklamakan Desert, forms a strategic buffer zone between majority of China's population and the Russian "empire" (past, present, and future). There are only a few routes to get to China. Goingthrough the desert is stupid and probably suicidal. Note that two major routes converge on Kashgar (喀什市). Due to geopolitical considerations, this city must remain Chinese, and is non-negotiable.

Speaking of routes... 

Before the Russians expanded up to the Chinese frontier, the area now known as Xinjiang was occupied to safeguard the passage of travellers on the Silk Road routes, since the Han Dynasty (~200BC to ~200AD).