I would say the one of the primary reasons China occupied Tibet in the first place was to display to the World the superiority of the newly created communist regime that had come into power. One has to remember that China broke all prior treaties which had been created between the rulers of Tibet and those of China after the communists defeated the nationalists and won the civil war that had started in China, in 1950.
The reasons for the continuation of Chinese occupation include the discovery of large deposits of natural resources in Tibet which include copper, lead, zinc, etc.
The glaciers in Tibet also form a large resource of water which is essential for China and also acts as a leverage against India, the largest rival of China. Some of India's most important rivers originate in the glaciers of Tibet.