Superman, Thor and Batman wear capes. Given that Superman can fly already, and Batman and Thor do not fly, I must rule out the reasoning that capes add aerodynamic functionality (In fact, all that flapping about and entangling of arms must cause some aerial and war-hammer throwing issues).
With that presumption gone, I come to only one conclusion about the wearing of the non-functional large item of clothing – the superhero cape:
Superman, Thor and Batman fear their butts look big from the back.
Given this, I must make the link that Superman has even bigger psychological issues with body image than others. I conclude this from the following:
Typically his Superman suit is so skintight that it can be hidden under his normal Clark Kent everyday-wear.
But his cape can’t be shoved down the back of his shirt without a telling bulge, so my next presumption is that it must be screwed up as small as possible (and made of crease-resistant material) and stuck down the front of his pants, ready for any phonebox.
This instalment of Superhero Presumptions brought to you by other notable cape wearers: Captain Marvel, Drs Strange and Doom, Robin, Supergirl, Batgirl, Storm, Magneto, The Cape, Shazam, Power Girl, any early-day Vampires, Red Riding Hood and Elvis.