As shown here, pressure decreases to less than 25% of ground-level atmospheric pressure when we get to an altitude of about six miles. Since this is a common cruising altitude for commercial airplanes, pressurized cabins are needed for the comfort and survival of the passengers. This is because the density of the air decreases along with the decreasing air pressure, so there is less oxygen in a breath of air. You've probably noticed this effect in high-mountain areas, as it often feels harder to breath.
Consider the following text:
Mark and Jordan want to take a trip together. Mark wants to go mountain climbing while Jordan wants to go to a concert out of state. Jordan sometimes has a hard time breathing as well so they take this into consideration while making the plans for their trip.

Which location should Jordan and Mark go to considering Jordan's health, the concert or the mountain?