Why a high haemoglobin concentration in the blood is necessary for people living at high altitude?

I read this information from PRE-U TEXT STPM BILOLOGY, why a high haemoglobin concentration in the blood is necessary for people living at high altitude? It is because people living at high altitude face the problem of low partial pressure of oxygen in the atmosphere. Loading tension in lungs decreases. Less oxygen is taken into the lungs, lowers the concentration gradient of oxygen between alveolus and blood capillaries in the lungs. Haemoglobin is not fully saturated. There is less oxygen carried in the blood (hypoxia). Less oxygen is supplied to tissues for aerobic respiration. An increase in haemoglobin in the blood is an adaptation to maintain oxygen carrying capacity.