100% N2 vs. 78% N2
Air is 78% Nitrogen, 21% Oxygen, and 0.9% Argon. The balance is a collection of many other gasses. You do not want to use air in suspension systems or track tires.
Why pure Nitrogen
Nitrogen is simply better than air in all respects when dealing with tires and shocks. Since N2 cannot hold water vapor like air can, the pressure change due to heat is very consistant.
Also, Nitrogen molecules are larger than Oxygen molecules. Less pressure is able to leak from the system due to this geometry.