Costumes are an interesting concept. I can't think of another life-form that intentionally adopts an entirely new persona, for motives other than survival. But then again, a large part of humanity is doing things for reasons other than survival.
Only in a rare case does a costume provide any form of practicality in the conventional sense of the word, the sense that it provides utility--and if it does, it does so in the form of identity protection. Costumes worn for identity protection will prohibit the true persona of the wearer from being discovered. These costumes, and most costumes in general enable a person to escape traditional social limitations, to push boundaries of acceptability, whether it be in violence, sexuality, or personal psyche.
Beyond practical identity protection, the functionality of costumes is limited. True, costumes are used for seduction or sexualization, enabling the wearer to adopt a persona uninhibited by the aforementioned social constraints, as is the case with fetishwear, however, the actual practicality of the costume is limited, as it is largely a psychological change, in spite of a change in fabric.
Even less practical is the use of a costume to adopt a persona that is socially convenient (if you want to get me going on my rant on the modern state of Harry Potter culture, I could give you an excellent example of such a "costume". First of all, a tie over a t-shirt is not a costume in this situation. Second of all, it's blue and bronze, not blue and silver. Quit your un-canon nonsense). In fact, whether this form of clothing change even constitutes a costume is questionable. If the individual persona is changed with a crowd shift in the mass persona, is it really change? Although the individual is altered relative to its initial persona, the individual experiences no change relative to the change in the masses. We're looking at net displacement and rate of displacement here. Physics, people.
Conversely, costumes are used to establish oneself as an independent entity or to attract attention. This is particularly common in subgroups based on nonconformity, or even in some forms of high fashion in which the goal is to be unique and ahead of the crowd in physicality. An individualized costumer wants attention to focus on the person in the costume rather than the costume itself, but will do so through creative means, often seeking more obscure costumes to disassociate from the typical.
The final form of costume I personally find most admirable in that it is completely independent of all social standards, save perhaps the pop culture with which we're constantly inundated. It offers absolutely no practicality. There is no purpose to it. It is the costume of a child--"spiderman syndrome". No identity is trying to be concealed. There is no goal of seduction. No social standard from which there is an urge to conform of depart. It's simply a whole-hearted admiration and aspirations of emulation.
An interesting anecdote on costumes: they differ from an entirely new identity. Identity is psychological. Costumes are physical--a camouflage. (Let it also be noted that camouflage is one of the most impossible words for me to spell. I had to type that into Google three times.) Often, a change in physique accompanies a change in psyche, but the two are not mutually inclusive.
Anyhow, I was at DragonCon in Atlanta, GA this past weekend. And being exposed to all of the superheroes and costumes got me thinking about identity. I suppose I should reward you for your patience with these puppies: