I use them when I just have a few items and I'm in a hurry.
Few things infuriate me more than other customers ahead of me in the queue telling the cashier their life story, then acting surprised when it's time to pay, and slowly patting each of their pockets in turn until they eventually locate their chosen method of payment, points card, etc. then KEEP TELLING THEIR POINTLESS STORIES after they are done paying and stopping me from moving forward with my trolley. I know it's probably a disproportionate reaction but it drives me bonkers! 