So, I'm sitting at school with the hordes of girls with their pants tucked into Uggs (okay, seriously, who ever thought that pajama pants tucked into Uggs was a good idea? honestly, guys) and it occurs to me, "why dress well?".
For starters, if you're in a minority that's often associated with one image or has a negative stereotype attached to it (like being black, no offense), dressing well can be the difference between being overlooked and being taken seriously. My friend Matt is Belizian and he once told me that his father told him that he'd have to look good all the time because he has to compete with white guys for girls. I wasn't thinking of girls at the start of this paragraph, but it's true. And Matt does dress really well, so he must have listened.
People take you seriously when you dress well - if you look frumpy and unconfident, people will assume you're frumpy and unconfident, that you're not sure about what you're talking about. Clothing, as Mercedes Lackey wisely pointed out in One Good Knight, is armor. The queen dresses lavishly so that people underestimate her intelligence and she can totally pwn them. She also uses her daughter's clothing to make people NOT look at her daughter, and also uses it to throw her daughter off-balance; her clothing has changed, so she's less comfortable and confident in herself.
You've heard of "dressing for success"? That applies everywhere, not just in business. Obviously, people dress their best at an interview, whether for a job or for a college, but people judge you everywhere you go. I'm a highschool student, so in general, people are going to discrimate against me wherever I go, people are going to have an eye on me because people in my age group aren't always... prudent in their actions. And if I look like a hooligan, it won't help matters at all. There used to be a drug store right near my school that would make all students leave their bags by the door so that we couldn't steal things - they wouldn't dream of telling an adult with a huuuuge purse to leave her bag.
The fact of the matter is that if you take the time to dress well and don't look like a hooligan, people won't treat you as if you're going to do bad things and judge you right off the bat.
People who dress well are, right off the bat, more attractive than someone who doesn't. If you are well dressed, you won't constantly be fussing with your clothes, and you'll be more confident, knowing that you look good.
So I guess the moral of the story is: Dress well so that people will like you more.