Nitrogen doesn't expand and contract like regular air. For every 10 degrees of temperature change in the air or on the tires themselves, the tire psi will change by 1. Cold it goes down, hot it goes up. With nitrogen it doesn't do that as much. Also tires can lose pressure over time through seepage when filled with regular air. The manufacturers say up to 5 psi a month, but that number is extreme. Nitrogen won't do this as much. Overall it's a scam for people who don't know shit about cars.the tire needs to be purged of all air before adding the nitrogen. Should you need to add pressure to a tire afterwards, better go and try to find a nitrogen fill in the fly, because if you add regular air, you have to start all over again. Meanwhile, the guys at the dealership rub their nipples as they see you drive away.