Tires are typically inflated with air that’s a combination of roughly 78% nitrogen (N2), 21% oxygen (O2) and 1% other gases along with a modest amount of moisture depending on relative humidity. Since all gases expand when heated and contract when cooled, tire inflation pressures rise and fall with changes in temperature by about one psi for every 10° Fahrenheit. In addition, the tire will flex during use (or even at rest) and allow air to escape through the rubber liner of your tire at the molecular level, losing as much as one additional psi of pressure each month. While that doesn't seem like much, if your tire pressure is ignored over a long period of time the result can be a dangerously underinflated tire.
In an effort to minimize these effects, many service centers and dealerships offer to fill tires with pure nitrogen, which has a different molecular structure that's less likely to escape through the rubber liner over time and has no ability to retain moisture. While your air pressure will still vary based on temperature, the changes are more consistent due to the absence of moisture. This would be most beneficial for drivers using their vehicles in a track environment where tire pressure needs to be very precise.
We often get the question "Is nitrogen worth the money?" If you have the ability to fill your tires with nitrogen at no additional cost, and if the initial cost of the switch from air to nitrogen is minimal, we believe the benefits mentioned above are worth it. We have found that in most cases our customers find it difficult to find stations that allow you to top off your tires with nitrogen every time. Diluting the nitrogen with standard air over time will greatly diminish its benefits (and decrease the value of the service).
To gain a better understanding on this topic, take a look at "Clearing the Air About Nitrogen Tire Inflation."