I believe in comics it goes back to Flash GordonWhich makes it even more arbitrary. Rephrase: Why would you want to cover the sides of your head and your neck, but not the top of your head? Artistically, I'd guess it was because comics artists got tired of drawing cowls and decided to mix it up by adding hair. But as something people would actually sew out of cloth and put around their heads, it's a pretty weird fashion concept, and as far as I know, it doesn't have much of a real-world precedent.
