I’m not sure if this is the right place for this topic/question, so if someone wants to move it please do.
It’s come to my attention that some debit cards store transaction logs on the card itself. Apparently Starling Bank cards do, but Monzo cards don’t. It doesn’t overly concern me except that anyone with an NFC reader can see these transactions. However, I am wondering what the use of storing the transactions on the cards is in the first place?
Here was one suggestion just yesterday:
Well, as far as I know it doesn’t actually say where you spent the money so even less use for that particular activity 
I wonder if it’s used for the offline limits on offline-preferring cards, to let the terminal decide how much of a risk they are taking. If there’s only one offline transaction prior to the current one then the risk is tiny, if there are 10 maybe it’s a good time to go online just to check it the user still has enough money?
There seems very little information out there about this, unless I am looking in the wrong place. So I asked Starling Bank via DM on Twitter why they store the transactions on the card. They said they don’t store the transactions on the card. It would seem that it isn’t widely known about within Starling Bank either! 
I’ve seen a Starling Bank debit card read with an NFC reader and reveal the transactions, so I know they are there.
However, I also read somewhere that some banks block the transmission of the transactions log via contactless, so now I am wondering if all cards do this but some are just blocked from being read.
There must be a reason for this, but as yet I can’t find any definitive information about why the transactions would be stored on the card and for what purpose.
It would seem that it isn’t widely known about within Starling Bank either!
They’re most likely using off the shelf card firmware and it has that feature built in.
I wonder if it’s just a legacy feature. I remember around 2005 back in France, there were off the shelf card readers sold in stores that could read the data, I actually had one to play around with.
Googling around I just found this article talking about them - the first paragraph reads “how to keep your accounts up to date without crumbling under piles of receipts/invoices?” which seems to suggest that the main purpose was for keeping track of your spending before online banking was a thing.