What I think the superrmarket is really doing is using deposits as a way to take advantage of the psychology of sunk costs. Since I have already set off a sum on their account as my food budget - payable with their card in their stores - I am more likely to treat shopping at their store as lower cost because it comes out of my already set aside food budget instead out of my general budget. Plus they offer discounts for having the card, for paying with the card, for using the card a lot and so on. And of course they are going to collect data from purchases to tailor offerings and see who pays what with what.

So the point with the deposits is not really to make money of them directly, but to use it as part of building customer loyalty around a payment/discount card.

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