You may well ask. But some people choose not to activate the switch function on their cheque cards, and find it easier to keep track of their spending using their cheque stubs, and anyway the nice machine at the checkout also prints the cheque out (well not all of it, but it was too nice an alliteration to pass up on).
Also, not all shops are geared up to take plastic. It's cash or cheque. I was in a pub earlier this week when a punter casually enquired, before ordering his pint, whether he could pay for his food and drink by plastic. "No," he was told, "but a cheque will be fine". At that very moment another punter was just paying by cheque, it must have been for twenty quid because I'm sure I saw a tenner in the change.