Airplanes often fly at great altitudes, of 10 kilometers or more. People would have serious trouble breathing at that altitude.
To keep water from leaking in.
Airplane cabins are pressurized because humans cannot breath at a very high elevation.Aircraft are pressurized to allow them to fly higher. Without pressurization, the amount of oxygen in the air would not be sufficient for humans to breathe.
Pressurized Cabins
Pressure is created in the engines and/or APU as "bleed air" and enters the cabin.
Helicopters don't generally fly high enough for cabin pressurization to be necessary.
Airplanes have broken up in flight after the windows failed.
Because at altitudes above 10,000 feet there is not enough air for humans to breathe. Aircraft cabins are pressurised so that the internal pressure is equal to that at about 7,000 feet.
Mostly not enough oxygen in the air at the low pressure up there.
To allow passengers to breathe sufficient oxygen, needed to survive, even in high altitudes.
Pressurized cabins on today's airliners were used on B-29 Superfortress's.
For two reasons: 1. To make the air breathable and comfortable. 2. In conjunction with the above, to keep the temperature at a safe level.
At altitudes above 10,000 feet there is not enough air for humans to breathe. Aircraft cabins are pressurized so the pressure is equal to that at about 7,000 feet. Commercial airliners fly at altitudes up to 40,000 feet.