Hubby and I normally travel by car but we wanted to have our passports ready in case we get the opportunity to travel abroad. The passport process can take several weeks, so if you ever need one quickly it might be too late. Also, I thought it was a good idea to take care of the process while we had the money to afford it. If we do take a trip later, that will be one less expense to figure into the cost.
That's just my reason. Everyone is different. :P
