If you are a superhero (whether main hero, Sidekick, or even just part of a heroic army), odds are you wear a cape.
Okay, not really that likely anymore, especially since the Silver Age, but capes are a typical thing for superheroes.
The reason for the cape goes back to the first Comic Book Super Heroes. Most prominently, Superman's outfit specifically mimics that of early 20th-century circus [...] [tvtropes.org]