What essential provisions do domesticated yaks provide to local communities in harsh climates?

Milk, meat, hair, and transportation.

Domesticated yaks constitute the vast majority of the global yak population and serve as absolutely essential livestock for the local communities managing them in the severe, high-altitude climates of Central Asia. Their utility spans several critical domains necessary for sustaining human life in these harsh environments. Specifically, they are managed to yield milk, which is a vital source of nutrition; they provide meat as a primary food source; their hair is used for various needs, likely including insulation and clothing; and perhaps most practically for movement and hauling, they serve as a means of transportation across difficult terrain.