Well, to begin with, Tibet does have a lot of valuable natural resources that have never been seriously exploited. Further, there are huge areas of China outside of Tibet proper that have large Tibetan populations. Controlling Tibet itself is seen as important in maintain control over these people. Moreover, the Yellow Hat school of Buddhism which the Dalai Lama heads is the tradtional religion of many people within China who are not Tibetan including the Mongolians and Manchus. Controling Tibet is seen as a means of controlling this and other forms of Lamistic Buddhism which have adherents in China. Finally, Tibet is one of many countries that have historically been part of the Chinese polity and the Chinese government has consistently attempted to re-assert its domination of all such countries with varying degrees of success.