I think it was in college that I developed the habit of dressing up before important presentations and tests. While my classmates rolled out of bed and went to class in sweatpants and slippers, I felt like the better I dressed the better I performed. There was a correlation between looking good and doing good.
I have a mentor who is extremely passionate about dressing well. For him, taking the extra time to iron your shirt or shave your face says a lot about what type of employee and leader you will be. Plus, the axiom holds true - you never get a chance to make a first impression - so why screw up your career because you were too lazy to dress well.
Contrary to the old Canon camera slogan, image is NOT everything. Things like integrity, honesty, reliability, and results matter. But we shouldn't be foolish, image does matter, both in what people think of you, and in what you think about yourself.
Try an experiment someday in the next week. Get up on a non-work day and put on some nice clothes. Shave your face. Do your hair. Leave the sweats in the dresser. And see how much more productive and successful you feel at the end of the day; if nothing else I guarantee you'll feel different about laying on the coach with a bag of Cheetos's spilled in your lap.