Superman's cape is part of his iconic costume, and it carries a symbolic meaning. It's part of the clothing he travelled to Earth in and, according to early stories, functioned as a giant pocket to keep his civilian clothes in while saving the day. Batman's cape carries more functionality, acting as a glider and a shield.
Capes have become so integrated into pop culture that they appear everywhere without being questioned. That's why we end up with these bozos flaunting about in capes for no damned reason.
Going just from Age of Ultron it would appear that Vision got is awesome gold cape because he saw how well Thor was rocking the look. But Thor has an excuse, it's part of his regal attire and carries cultural meaning. Vision doesn't need a cape, he doesn't even need the aerodynamic aide when flying...he just wants to look cool.
In more recent incarnations the Boy Wonder has had extra function added to his cape, but this wasn't initially the case. It was a purely ornamental strip of fabric that added a tiny bit of modesty to that original outfit. Dick Grayson, the original Robin, took not of this when he stepped into Batman's boots, claiming he got rid of the cape as soon as possible because it interfered with his movement.
Vader likes to be imposing. That's why he has extra high heels added to his space boots when he got locked into that costume. For reasons not entirely apparent he added a cape to the mix, possibly to make flying his TIE Fighter a bit more awkward. As they pointed out in No Such Thing As A Fish this week, things must've gotten interesting in zero gravity. Special mention to Lando, who copied the look.
Most Jack Kirby characters only sported capes if they were functional or part of royalty - like Thor and Doctor Doom. Magneto is an exception to this, because he wears the hell out of that cape. It should be noted that Magneto does everything with extra flair, and a cape certainly fits that criteria. I mean, this is man who signed his name in cursive when putting a giant, threatening message made of bullets in the sky.
Let's face it, nothing this guy wears makes any sense. His abilities comprise entirely of illusions and magic tricks, he could be wearing a pair of jean shorts for all it matters. But he is a showman, and for some reason this means he should wear a cape a giant bowl on his head. Dummy.
Right, this is opposite of functional. When you spend all your time floating around and making big hand gestures in order to cast spells a giant, heavy cloak is just going to be a nuisance. It's in the way of your arms and is just asking to snag on something when bobbing around the Sanctum.
When Steve Rogers hung up the shield as Captain America he took the name Nomad and redesigned his costume to show more pecs. He also went against his military background and added a fabulous cape to the ensemble. He regretted this decision the first time he went running into battle and tripped over the blasted thing.
OK, what the hell is that thing? It barely qualifies as a cape, at least not as one that hasn't been shredded. There's no way that silly bit of glam isn't annoying as all hell. Imagine what happens when the wind changes...you're gonna get a face full of not-cape.
Yep, we're back to Star Wars. Darth Vader's cape may be pointless, but Boba Fett's is downright dangerous. He gets around using a JET PACK people. It's just asking to go wrong. It gets caught on a branch during take-off and he's got a snapped neck. One wrong move and he's set himself on fire. Sudden change in direction and he's blinded as it flips over his face. Jetpack and cape combo is asking for a death scene less dignified than a trip down the Sarlacc Pit.
C'mon, you were expecting it.