CASH SALES ON BORROWED TIME IN BRIGG SHOPS

The number of cash transactions in Brigg shops continues to fall as more and more folk elect to use cards for their payments at the tills - particularly the younger element. 
Similarly, good old cheques are being accepted by fewer and fewer outlets these days.
It's something of a paradox that people who choose to pay for their purchases by card are invariably offered "cash back" by shop assistants.
Can someone explain the logic behind that? You choose not to pay with cash but then leave the store with some notes in your purse or wallet. 
Paying by card is slower than handing over cash at the counter and can hold up other people in the queue. But at least it gives you time to rummage through your pockets and get the necessary coins and notes ready for when it's finally your turn to pay.
Here's an idea for larger stores: Let's have a designated cash payment-only till at the checkouts.