These loyalty card things are used by marketing bods to keep an eye on buying trends and whatnot and so they can target advertise to you. I have a card that I only ever use to buy completely random single items, for example in Wales we get charged 5p for a carrier bag, so I might buy a load of food at Tescos and only use the card for the purchase of the 5p carrier bag, I might do this for 5 or 6 times, then randomly just buy a tin of beans with this method in the futile hope it skews the 'marketing machine' up in some small way.