On the data thing...
Supermarkets use the data to help improve what they offer their customers. It's analysed to understand who (as in demographic group, not at an individual level) buys what, at what kind of frequency, in combination with what, and on what kind of shopping trip (small basket vs big trolley). They can see what's changing over time and act/adapt accordingly. It means they can plan promotions better, merchandise in the most effective way, use it to inform planning for future NPD and much more.
They want us to spend as much of our shopping budget as possible in their store (they are well aware most people shop around), so it's used to try and retain customers/get more of our weekly budget (not nec to spend more in total, but to spend more in their store).
For example... let's say sales of sausages have dropped. They can look at data and understand who isn't buying them any more, what they are buying instead, whether their spend is the same but just not on sausages (maybe they're buying more fish fingers, or bacon, or veggie sausages...) or whether they are spending less overall/with different shopping patterns (so maybe buying sausages elsewhere). So they can do a much more informed job of answering the question of 'why have our sausage sales dropped and what are we going to do about it?' than if the only data available was the headline sales figure.
If supermarkets didn't have loyalty schemes, they would be much less efficient, and prices for everything would be higher.