go to www.peopleofwalmart.com and then kick yourself for asking this question.
To fit in with everybody else. So you will not stand out from the crowd. So you will, like Henry David Thoreau said, live a life of quiet desperation.
so that you don't feel like a fish out of water. nothing's worse than being dressed inappropriately for an important event - you'll be remembered for the wrong reason.
Of course it is important to wear appropriate clothing to certain places or for certain occasions. Usually there is an acceptable normal for most occasions, and that is what people go by. One wouldn't wear pyjamas out to a 5 star restauraunt any more then one would wear a ball gown to a pyjama party. Everything is acceptable within certain perameters. If you are going to a fancy Christmas party, for example, it could be anything from nice dress pants and a fancy top to a full length ball gown. Dresses, skirts, pants and nice shirts are all exceptable. There is nothing more uncomfortable then showing up in blue jeans and a t-shirt when everyone else is wearing dress clothes. Simarilarly, there is nothing worse then showing up in very dressy clothes when everyone else is wearing boots and jeans. Check with your hostess if in doubt as to the dress code. =)
because there's time and place for everything. For work, you wear business suits or maybe business casuals; for weddings, you wear formals dresses, and fancy suits; for gym, you wear sports clothes; and for shopping, casuals like Jeans.
simply to show respect to ourselves and to others...
Because people like other people to "fit in" -- gives them a sense of security, of expectations of behavior
There are many reasons. First, being practical. Wear high heals when climbing a mountain, no one will judge you, it will make the job impossible.
Second,express the way you feel inside. If it's your wedding day, wearing ugly cloths will definitely show you don't want to get married(I didn't mention anything about unconventional outfits, as long as they express what you feel).
And maybe there are more valid reasons, but I don't know them.
Society dictates it.
I think clean & neat should be the only requirement, myself.