Marketing a product is about doing one of two things: it's either about selling someone a way to follow whatever narrative they've picked for themselves, or selling them a way to try on someone else's identity for size. Although this doesn't apply to most good that aren't purchased by consumers, it's a great guideline for consumer products, especially clothes. And hooded sweatshirts are a great example.
Hooded sweatshirts offer wearers a chance to adopt multiple personae. The most obvious is that of someone who is young, hip, and too busy to be considered rude, but otherwise fairly brusque. Anyone in New York can see plenty of examples of this specimen walking down the street (often, though not always, listening to an iPod). The clothes won't turn you into somebody else, but they will allow you to act like someone else for a while -- or if they don't let you act like someone else, they'll let you look like someone who acts like someone else, and be treated differently because of it.
It's easy to wonder why simply donning hooded sweatshirts would make someone feel that they're playing a different role. There are a few good reasons for this: since clothes affect expectations, picking clothes of a certain kind will make people likely to accept certain behaviors; someone in a suit is unlikely to say "ain't," and someone in a backward baseball cap is unlikely to say "indeed." Picking up some hooded sweatshirts doesn't make you change the language you use, but to avoid shocking people you might end up talking and acting a bit like the average wearer of hooded sweatshirts.
This is not a problem, of course. Being able to pick up and discard roles at the drop of a hooded sweatshirt is a nice freedom to have. It's the chance to relate to other people, to experiment with other perspectives, and perhaps to gradually change one's own personality and views. Since our personalities and behavior are already shaped by outside expectations -- which may have nothing to do with our internal inclinations -- the freedom to adopt a new outlook with hooded sweatshirts is a useful way to reclaim another freedom: the freedom to decide who you want to be.
Hooded sweatshirts don't directly change behavior -- there's nothing intrinsic to the sweatshirt that will make someone behave differently. Some people expect to change their lives immediately with clothes -- it's better to think of clothes as a catalyst to increase the speed at which an existing change can occur. Buying hooded sweatshirts won't make you a different person, but it might just help you be the different person you meant to be all along.