Capes have the obvious effect of making the wearer look larger and more imposing. Thus artists often use them as a way to make characters look Badass. Just look at Batman over there. He just wouldn't look half as awesome without his cape. But the main reason for this is that in the days of circuses, the strongmen often wore tights and a cape, thus the association with toughness in characters. The trope was then grandfathered into the present day by Superheroes. This can apply to any character, whether it be a hero, sidekick, villain, and it doesn't even have to be a superhero setting.