You see two businesses in front of you that do the same thing. One business has people sitting around in shorts and t-shirts and the other has people dressed up in formal attire, shoelaces tied, and the people are properly groomed, etc. Without knowing nothing else about the business -- which one do you think takes their job more seriously?

When interacting with customers, you must let them believe that you value them. One way to do that is to show proper attention to your appearance and behavior for their sake. That's why front-line and customer service workers must show the proper amount of attention and respect to their clients by being professional in appearance and mannerisms.