Gasoline (petrol) stations can sell them in denominations specifically designed to allow them to apply maximum gouge force on the buyer based on the 9/10 cent scam. They can also have immediate access to the money you spend on the cards, essentially given them an interest free loan with a promise for future gasoline (petrol).

The buyers in the mean time will have a convenient card, capable of "storing" fuel for times of need, this will essentially allow the buyer to "hoard" fuel, and perhaps sell it themselves at a higher premium later on, much like stocks, without the typical risks inherent in large stockpiles of flammable liquids. Also, since the gas cards exist, it may behoove the stations to keep the prices fairly constant, rather than increasing them by leaps and bounds each summer. The cards can likely also be made to work in the existing ATM slots found in many gas stations, without the need for typing in personal information, which could help to cut down on identity theft, and otherwise help to protect the privacy of people who so desire.

Promissary cards of this nature could become a lucrative business for speculation, and ten years from now, when things are starting to progress poorly, a powerful EMP, or a massive giveway of eelskin wallets would likely erase the data on most of them, thus clearing the gas companies of any need to repay on their promises. It's a perfect model of American Business at it's finest.

Oops. Pretend you never read that last paragraph there.