Symbolism, Imagery, Allegory
If you see a helmet in this novel, chances are good it's a symbol for the triumph of the imagination over reality.
Just as a helmet gives you tunnel vision, so does Don Quixote's obsession with knight-errantry make him unable to see or acknowledge anything that doesn't fit into his narrow worldview. When he first sets out on his adventures, the Don realizes that he needs a proper helmet, but the book tells us that "instead of a complete helmet, there was only a single head-piece: however, his industry supplied that defect; for, with some pasteboard, he made a kind of half-beaver, or vizor" (1.1.1.9). Don Quixote doesn't detect the silliness in making part of his helmet out of cardboard, because his crazy imagination only allows him to see what he wants to.
Don Quixote's diseased imagination makes him mistake another household object—a barber's basin—for a helmet. As the book tells us, Don Quixote sees a barber riding toward him and "[takes] the barber for a knight, and his brass basin for a golden helmet" (1.3.7.2). Sancho tries to tell him over and over that the object is actually a basin, but Don Quixote has already made up his mind. The object is a helmet because, according to Don Quixote's imagination, it has to be.
In nearly every instance when the question of helmets comes up, Don Quixote's imagination always trumps good sense. And this just goes to show you how skilled the mind is at throwing away unwanted information and only accepting the things it wants to.
A final question to ponder, though: do Don Quixote's silly helmets actually work? Do they protect him? If so, does it matter what they look like if they do the job?