Many credit cards offer 3-5% rebates on gasoline purchases. If you are paying for your gas with a card anyway, signing up for a gas rebate card will put a few bucks back in your pocket...
If you fill up your 15 gallon tank once a week, that could be over $150 a year back in your pocket. A pretty easy way to save money.
These cards may also offer a rebate on other automobile related purchases like oil changes.
Some cards even offer benefits like the travel accident insurance, auto rental insurance, and various fraud and security protection services.
Just don't forget to pay the balance in full each month, or you'll end up paying more in interest than you save in rebates.