One reason is to add a foil-faced layer of insulation with the foil facing the *outside*, just under the external steel bus skin.
We just had our hottest day so far -- 97 degrees outside in southernmost Arizona. The sun altitude was around 57 degrees @ solar noon....and our bus was only 2 degrees warmer than the outside air temp. Not bad for what is essentially a steel tube with no A/C or evaporative cooling.
There are other factors...white paint, east-west bus orientation, and having every door & window open for max air flow. Our two doors are on the north (shaded) side of the bus, so the coolest available air is moving through the bus.
Hope this helps