Another of the reasons for using nitrogen ( I was always told this was the main reason), is that a nitrogen molecule is approx 4 times the size of an oxygen molecule, and as rubber is slightly permeable, the leakage rate is far less. ( I know air is only about 22% oyxgen, but that is still enough for leakage to be a problem).
This leakage isn't really noticeable at ground level, but once you get to high altitude, it then becomes more of a factor, due to the higher differential between the internal pressure of the tyre and the ambient pressure.