Where I live, we don’t yet have those cards. I despise the idea of them and hate getting caught out by them (I’m looking at you Tesco and Lidl).
Normal price for something - 0.49 (in stores without these cards) But in their store it’s only that price with their ‘club card’ or ‘plus membership’.
Otherwise it’s 1.59
There are loads of other items with high markups.
BUT - the marked price is the normal store price to the inexperienced shopper of these data collecting stores, with the blind spot being the need to have a store card to get the ‘discount’. My full shop was an extra 103.46 altogether between the two stores (51.22 in one 52.24 in the other) rushing to get christmas dinner and dining and extras. Should be banned. /rant
The government doing a sick job of protecting consumers. /s


They do this because it allowed them to track how often you shop and what you normally buy. This helps them to manage their stock, keeping popular items that regular shoppers buy in stock etc.
The way it normally works though is that the item is $1.99 but if you have their club card or member card the item is $1.50.
This means you save money in exchange for allowing them to track your shopping habits.
Don’t get me wrong. There are a lot of … We’ll call them questionable reasons why business want to track your shopping habits, and that tracking doesn’t necessarily stop as just tracking what you buy.
But it was never meant to be item is less expensive but you only get the less expensive price if you have their card. It was supposed to be, we’ll give you a deal on said item if you let us track you in exchange.
Nope. Normal stock management works without tracking individual users habits.
These cards have the benefit of binding customers to your chain (and to sell ads).
They can track what people are buying just as well without needing to know who is buying what. If all they cared about was making sure they are stocking items that are popular with customers, they could just monitor how many of the item were scanned at the checkout counters.
When you give your phone number to the store, it serves three functions: