I think they tend to be close to the shop so it's safer for children not having to walk so far into the car park. My pet peeve is lazy parents abandoning trollies in the pavement from the parent bays to our local supermarket meaning I have to navigate through moving traffic with my toddler and baby. I've always managed to safely get my shopping in the car, trolley back where it should be and kids back in the car without having to abandon children or trollies at any point!