While I can see a suit doesn't necessarily make a person better for a job, it does show a level of detail and committement and just what sort of person that person is. I think that's harder to see in casual clothes. For instance, does the suit fit right? Does it all go together (no brown shoes and black pants...)? If a person pays that level of attention to how they dress, then most likely they'll pay attention to the details of a job that they do.