Even the music that is piped into your local grocery store is designed to make you spend and shop more. Supermarkets and shops in general tend to play slower music which encourages you to take your time and walk at a more leisurely pace. This then enables you to discover and explore more, and subsequently put more in your basket.
Also, after around 23 minutes of shopping, we start to make more emotional and less practical decisions, so when we may have told ourselves at the start that we didn't need that chocolate, by the end we'll feel like we do.