Why is China so interested in controlling Tibet, a desolate land where nothing grows, and with a tiny population of only 2.5 million inhabitants on a land bigger than Germany, France, Britain and the Benelux combined ?

It could be argued that it is for historical reasons. Until the 7th century, Tibet was a barbaric region, often raiding China, as did the Mongols. In 641, the Chinese emperor decided to marry his daughter to Tibetan King Songtsen Gampo, and thus convert him to Buddhism. Since then Tibet has become a much more peaceful nation, and a fief of China according to the Chinese. It is important to remember that Buddhism was imported from China, and thus all this aspect of Tibetan culture is Chinese in origin.

It felt natural for China to claim Tibet as part of their country. The atrocities committed against the Tibetans by the Chinese can be explained by the extreme reluctance of the Tibetans to comply with the Communist authorities, contrarily to the rest of China. Whereas most Chinese people readily threw away their old customs, religion, clothes and habits like the government told them to do, the Tibetans resisted with an unequalled stubornness, preferring to die rather than to adapt. This obstination is what I believed caused the most harm. The Chinese didn't behave so differently in Tibet than in the rest of China. Maybe that's what the Tibetans failed to understand. It wasn't discrimination, it was a radical change for all Chinese people too. Many Chinese didn't like it better than them, but they understood that it was pointless to oppose Mao. The Tibetans reacted in a very provocative and unreasonable way.

China wanted Tibet because it considered it as part and parcel of the Chinese sphere of influence (which is rather true), but also for its geopolitical value. Controlling Tibet means having a border with India, Nepal, Bhutan and Pakistan. Nowadays, the border with Pakistan is essential to China to bring Middle-Eastern petrol. India being the 2nd most populous country on earth (and soon 1st), and the immediate economic and political rival of China, it is also vital for the Chinese to control Tibet for their own security, and maybe with future trade prospects in mind.