They put nitrogen in them , because nitrogen has no moisture in it. Moisture will corrode the wheel assymbly, causing reduced life of the magnesium wheel. Atleast all Air Force planes use nitrogen. If they need serviced at a place where nitrogen is not available, they can use straight air, but once they get back to a home station they must be drained and refilled with nitro.
At Pope, where not happy, until you're not happy!