What cause air pressure? Why do aircraft need pressurized cabins?

Air pressure is the result of the weight of all the air that's above you. (That's why the pressure decreases as you move up, and put some of the atmosphere below you.)

Aircraft need somewhat constant pressure inside for 2 reasons:

1). When the outside pressure drops, the airplane body stretches a little bit. When the outside pressure grows, the airplane body is squeezed a little bit. If allowed to go through hundreds of these cycles, the aircraft body eventually fails and falls apart. This was the cause of the great many failures of the first high-altitude commercial passenger jet ... the DeHaviland Comet.

2). Your lungs can't "pull" air in. All they do is make themselves big, and you rely on outside air pressure to blow the air into them.

If the outside pressure isn't great enough, then your lungs can make themselves big, but still not enough oxygen blows in to keep you conscious and alive.

In an airplane at high altitude, you need more pressure than what's naturally available from outside.

Gas expansion. As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. These changes may have effects where air is trapped in the body