We use a pressure cooker to cook at high altitudes because of the following reason(s).
It increases the pressure inside the cooker.
it increases the boiling point of water.
The temperature at which the vapour pressure of the liquid is equal to the pressure surrounding it is known as the boiling point. At higher altitudes the atmospheric pressure is low. So, water will boil at a lower temperature because lower the atmospheric pressure lower will be the boiling point of water.
The pressure cooker raises the pressure inside the cooker to the same absolute pressure regardless of the altitude at which the cooking is being done. So water will boil at a higher temperature, so more heat will be provided for cooking. A pressure cooker also reduces the cooking time.