Why did Sang Hongyang suggest that Emperor Wu of the Han Dynasty reclaim land for four years (the first 89 years)?

In the fourth year of Zhenghe (the first 89 years), Sang Hongyang wrote a letter suggesting to reclaim wasteland (now luntai county, Xinjiang). Luntai, located in the center of Tarim Basin, was the only place for envoys and caravans of Han Dynasty to sail to the West. Yanqi, Shu Wei and Yuli (all in present-day Xinjiang) to the east of Luntai are the areas where Xiongnu servants and commanders are often stationed. Sang Hongyang believes that opening up farmland here can not only open up more than 5,000 hectares of fertile land, develop agricultural production and increase national treasury income; Moreover, it is also of great significance to further crowd out the Huns' forces in the western regions, defend the traffic between China and the West and the security of the northwest border counties of the Han Dynasty.