Michael Chabon has a long essay in The New Yorker on the meaning of superhero costumes.

He writes:

In theory, the costume forms part of the strategy of concealment. But in fact the superhero’s costume often functions as a kind of magic screen onto which the repressed narrative may be projected. No matter how well he or she hides its traces, the secret narrative of transformation, of rebirth, is given up by the costume.

[…]

We say secret identity, and adopt a series of cloaking strategies to preserve it, but what we are actually trying to conceal is a narrative: not who we are but the story of how we got that way”and, by implication, of all that we lacked, and all that we were not, before the spider bit us. Yet our costume conceals nothing, reveals everything: it is our secret skin, exposed and exposing us for all the world to see. Superheroism is a kind of transvestism; our superdrag serves at once to obscure the exterior self that no longer defines us while betraying, with half-unconscious panache, the truth of the story we carry in our hearts, the story of our transformation, of our story’s recommencement, of our rebirth into the world of adventure, of story itself.

[…]

Talking, retying the knots of our capes, flip-flops slapping against the soles of our feet, we transformed not only ourselves. In the space of that walk to the pool we also transformed the world, shaping it into a place in which such things were possible: the reincarnation of an Arthurian knight could find solace and partnership in the company of a latter-day Mesoamerican wizard.

[…]

All we needed to do was accept the standing invitation that superhero comics extended to us by means of a towel. It was an invitation to enter into the world of story, to join in the ongoing business of comic books, and, with the knotting of a magical beach towel, to begin to wear what we knew to be hidden inside us.

Translation: costumes make nerds feel cool.