Scanning as you shop, with the store’s handset (or app, in Waitrose), and packing it straight into your own bags is so much quicker than unloading everything onto the checkout belt, letting the cashier scan it and you having to load it into the trolley again. You’re only handling your groceries once instead of three times. If you’re only getting a bag of shopping just scan and put them straight into your reusable bag, pay and go.
Obviously if you have bought age-restricted items they have to be authorised, and very occasionally the self-checkout will randomly request a partial or full rescan, it’s still quicker than loading your trolley, unloading onto the checkout, and loading it again.