This builds an illusion (or just builds) credibility. People are more likely to trust and work with more professional looking people. If one were to wear a t-shirt that said "SHREK" on it, and use language like they were writing a text message, it looks as if that person was like a child, had no work ethic, and little experience and credibility. However, if one dresses and speaks more professionally, it seems that they will have a better work ethic, and are more experienced and therefore better for the job.