Well, you are kinda right, it started in the aircraft industry (mostly military) and then transferred to auto racing. When you have a plane that takes off on a runway that is 150 degrees and within a few minutes is -30 degrees, there is a lot of shrinkage(that's what she said). The exact opposite is true on landing with even higher temps, due to friction. Nitrogen doesn't expand or contract like atmospheric air. If you can get the nitrogen for free, I would say go for it, personally I wouldn't pay $50 for it, I would check my tire pressures monthly like suggested and invest the $50 in oil futures.