I tend to rant about this sort of thing so ......
The main purpose of any loyalty card is to record information about when and how you spend your money. In the UK Tescos had it down pat where they would send you special offers 'tailored' to your 'needs'. This means they spend a lot of effort in working out how you spend your money and how they can get you to spend more with them.
The big issue is that they then share this information - you agree to it when you apply for the card - with other companies who will then plague you with their tat. There are also deeper implications - don't be surprised if your medical insurance premiums go up if you buy a greater than 'normal' amount of fatty foods or alcohol.
Big Brother is not only watching you, he is playing dirty.
Don't use these cards.
End of rant. I feel so much better

.