Question

Why are cabins of aeroplanes flying at high altitude are pressurised.

Answer 1

Pressurization is required for physiological reasons
Cabin pressurization is the active pumping of air into an aircraft cabin to increase the air pressure within the cabin. It is required when an aircraft reaches high altitudes, because the natural atmospheric pressure is too low to allow people to absorb sufficient oxygen, leading to altitude sickness and ultimately hypoxia.