Underinflated tires might cause an accident that could kill you. Need another reason for inflating your tires?
Underinflated tires increase tire wear, which could burn through a set of $400 tires a year early. Need another reason?
Underinflated tires waste gas. How much gas? The Department of Transportation estimates that 5 million gallons of fuel per day are wasted due to low tire pressure. That's more than 2 billion gallons per year, just because people don't take the time to inflate their tires properly.
OK, OK, you get the point. Inflating your tires to the specified pressure is important. So make it a habit to check and refill your tires once a month. And remember, you can't tell if a tire is underinflated just by looking at it. If it actually looks underinflated, it is way underinflated.