It puts them at ease so they can do their job accurately and efficiently. Maybe there's a bag fee and I don't have my reusable bags and they say don't worry about it. Maybe they make sure to bag my groceries without putting a gallon of milk on top of a loaf of bread. Maybe it's a restaurant I get food at periodically and they remember me being nice to them and they throw me a coupon or a free dessert. Maybe they were having a shitty day and just needed someone to look them in the eyes and see that they existed and making them feel better makes me feel better. Maybe they're really Morgan Freeman God in disguise and they're going to make me a deity for a day. I don't know man, it's just fucking nice. I don't need another reason.

Do you really ignore blue collar workers who are doing a job for you and just refuse to speak to them for no reason?