They do it because there's less oxegen in the atmosphere at higher altitudes. If they get used to exercise in oxegen deficient altitudes, it means at normal oxegen rich environments the exercise will be much easier ( for their lungs to handle anyway), thus leading to better performance.
It's a kind of train yourself for the worst so normal will be easier , if you get what I mean.
When the air is thinner, it takes more effort to preform activities due to the fact that there is less oxygen to inhale. When you become efficient at doing things in high altitudes then when you compete in low altitudes it will be easier, and you will not be caught of guard when competing in high altitudes.
Basically because there is less oxygen and your body creates more red blood cells to . When you travel so somewhere with lower altitude you keep those blood cells for a little while and therefore can run/bike longer distances without getting tired.
And because the air is thinner at higher altitudes, it works their lungs more. Once they return to lower altitudes, it is much easier to breathe, therefore they perform better.
Because the air is thinner in high altitudes.
Training where there is less oxygen the body creates more red blood cells to compensate.
to get elevated
