It's not commonplace yet, but some food marts have started putting TVs at checkout lines. The effect is twofold. These screens expose you to product ads, perhaps prompting you to buy more stuff either during that store visit or the next time you shop. They also distract you. You're probably not as focused on making sure the cashier rings up items correctly. Nor do you notice that your child snuck a magazine or wad of candy onto the conveyor belt that just got scanned and bagged.
It's not hard to get sidetracked or distracted from the task at hand while grocery shopping. Sometimes, it's easiest to stay focused (and on budget) by simply sticking to your list and staying attentive at the checkout counter.