What you wear tells something about your attitude. If you care to dress up very well for an interview, you will show a positive attitude as soon as you step into the room. Because it takes effort, good thinking and earnestness to dress up. When you dress up well for a meeting, you tell others of how committed you are to it.
Being neat is important in special occasions. You will give a negative impression if you dress shabbily or untidily. And visual impression can matter a lot. Once again being neat comes across as one who is caring, mindful and reliable.
Dressing up defines your own style. Dressing up is an art. That's why fashion runaways can be interesting to watch.