Why the batman needs a cape?

[SprangFan]

And why does Robin have a cape? And a rather small, fairly useless one at that.

Batman uses it to yank him back when he's too over-eager and rushes into danger. Sort of like a leash.

[Yalpsoc]

To possibly add more detail to the answer which has already been given repeatedly, the cape was worn (at least in-continuity) to enhance the resemblance to a giant bat. The scalloped edges of the cape, those delightfully flowing curves and points, mimic the shape of the sections of skin stretched over the bones in a bat's wings. Even on the TV show, we see him periodically holding up the sides of the cape, to provide a bat-like silhouette.

A cape could also be used to confuse an attacker about the size, shape, or body position of the cape's wearer. It could be used as camouflage, to hide more effectively in a shadowy or darkened area. Swoop it around, and it might be visually disorienting. Possibly it would be harder to aim a gun effectively at the vulnerable areas of a Batman who was creatively using his cape in one of these ways.

The Bat-capes are fun to draw, IMO. Those swooping, swirling curves and points can be used for decorative or compositional effects. The areas of light and shadow in the illustrated cape can be used to enhance an image.

Wearing a cape would also have drawbacks, of course. A long cape could easily cause one to slip or trip. It could get snagged on objects, knock things over and make unwanted noise, become trapped in doors. You could get sucked into that jet engine, as in The Incredibles. Wearing a cape like Batman's might take some practice.

Superheroes seem to be losing part of their history, particularly the costume design link back to those circus acrobats mentioned above. TV and film adaptations are changing the perception of superhero costumes. Kids these days, they don't get how wonderfully nifty superhero costume design has been, over the years. In those Golden Age years, costume design was all over the place, gaudy and weird, full of color and flair. I'm gonna walk away now, muttering to myself grumpily about "kids these days".

It seems that you did a lot research on this point. I like your ideas.

[chrisbcritter]

And why does Robin have a cape? And a rather small, fairly useless one at that.

Wouldn't that go with his having been an acrobat? Plus it's shorter because he was younger - like when boys in olden days wore shorts instead of long pants (remember the old saying "I knew you since you were in knee pants!").

[Riddler Fan]

I thought we all learned from the 2004 movie "The Incredibles" that capes can cause problems. Look what happened to Syndrome?

[Mr. Comic Book]

I guess that Superman needs cape because he will look handsome when he is flying, but Batman can not fly, why does he need the cape?

In most incarnations of Superman, the cape is part of standard Kryptonian clothing, or at least Krypton's ceremonial garb. It has zip-zero-zilch with wanting to look good in flight (especially since it's the first part of his costume to get damaged in battle) and everything to do with paying tribute to his native culture.

With Batman? Stiff faux-wings aren't an option (they were apparently a part of Bob Kane's initial character sketches) because they're too big and unwieldy. He needs something to sell the bat motif because...well, let's face it, the cape is the only truly bat-like aspect of his costume. Otherwise he's just some blue and gray guy with long pointy ears. Also, a cape made of bulletproof fabric would be suitable protection from gunfire, it would be a good way to shield innocent people from gunshots and falling wreckage, and it would be a good way to mess with his enemies because he can manipulate it in such a way that they can't tell whether he's coming or going. Where Superman has cultural reasons for dressing as he does, Batman's cape boils down to absolute necessity. It offers him protection and gets the bat-like image he's going for across, two things he wouldn't have otherwise.

Now obviously, you wouldn't want to wear a cape that's too long, too short, or too heavy. Christian Bale's cape (like the rest of his Bat-costumes) was absolutely ridiculous because it was too long to actually be useful. By the same token, Lewis Wilson's cape was too short for what Batman's costume is aiming for. Somewhere between the Adam West cape and the Keaton/Kilmer/Clooney capes would be the ideal length. Long enough to create the impression of large bat wings, but not so long that Batman can't run or fight in it. And a lot of times, it's going to boil down to how an artist draws it on the page.