As someone who is into cool motorsport stuff, the primary purpose of nitrogen in tires is because it doesn't expand or contract as much as air due to temperature change, which allows tires to maintain constant pressure when being subjected to the rigors of racing, which causes tires to heat up very rapidly to around 200 degrees. (More, if they start slipping and smoking) The more constant your tire pressure is, the better you'll perform. It's starting to trickle into more typical cars, because of its pressure stability, which reduces maintenance and improves fuel economy because the tires hold the right pressure for longer.