I'm waiting for a plane to Santorini and I overheard a conversation with a
dad and his about 10 year old son. The boy asked something, and the dad
replied:
"The air pressure at 10 km is much smaller than on the ground. The pressure
difference between the cabin and outside is decreased by having a decreased
cabin pressure, something like we have at 3 km. If this were not done, the
airplane structure should be more sturdy to sustain the force due to
pressure difference."
Is the explanation correct? I don't know much about aeroplane design, but
the explanations involves a valid physical idea.
At any rate, I quite like the way how the dad answered the boy's question.