SVeeedy Gonzales

Nitrogen is more stable than air alone. You'll experience less air loss over time (something to do with the size of nitrogen molecules being less easily passed through the rubber. Although air consists mostly of nitrogen, your tyres lose pressure because the other molecules - carbon dioxide, oxygen, whatever - pass through the tyre wall with greater ease) and there'll be less fluctuations in tyre performance during riding as nitrogen is less reactive to heat than air.

You won't get any real benefit from this or even notice anything unless you're faster than Rossi though, the gains are so small, even at levels of riding of top racers, using nitrogen is a tiny, tiny edge. But it's worth having to make people who put crappy old air in their tyres feel like they're out of touch.