Have you ever stuck your hand outside of your car window while driving on the highway? Remember the feeling of the pressure on your hand as the air collided with it. Now image, for planes traveling at hundreds of miles per hour how much pressure the fuselage or the wings would feel. The faster and faster these planes go, the more pressure is applied to the aircraft. Imagine the pressure experienced by hypersonic vehicles (ones that travel 5 times faster than the speed of sound)!
Aircraft also undergo thermal expansion and compression repetitively. They are parked in the warm hot sun on the runway around sea level and when they take off, they climb to altitudes where the static temperatures go down to -60°F. When metals are heated, they slightly expand and when they are cooled, they compress. Both pressure and temperature change affect the structures of the aircraft and can cause damage if not properly designed. That is why flight vehicle structures are important because if not properly designed, wings and fuselages can deform which would in turn cost a lot to fix and could prove to be dangerous!