Think about it. You want the person on the front lines to be happy, so they are happy to customers. You want them to feel comfortable , so they are comfortable with your customers. If you treat that person badly or with disrespect, how do you think they will treat customers when you're not there.
It comes down to this, if your people are happy, they are going to treat your customers well and spread happy thoughts and comments, which will in turn produce more sales.