A brand. A sign. A symbol, not just for a corporation, but for allegiance to a corporation. A sign that says, "I am a consumer. I am not an individual, I am a sheep. A mindless member of the flock. I am a number. Baaa."
It is a sign which, just like Calvin Klein, Old Navy, etc., is a blatant symbol shouting out to the world, "I paid more money than I had to for these clothes, just so I could wear this symbol to show everyone that not only can I afford to pay extra money for this, but that I'm one of the 'cool' people."
These brands are popular because people buy them. And because they are popular, people buy them. They have come to represent popularity, and to symbolize acceptance in "the 'in' crowd." They are a false definition of a person, worn by people too caught up in conformity to be able to define themselves in any other way.