Why would Superman need a cape to fly? Its all about style, capes look good on super heros. In fact, back when he was created, capes were very common and became cliche so he had to have one. Many characters do not have a cape now. But the cape just looks so darn dashing in the wind doesn't it?
In scouts a neck tie can be used as a brace for a broken leg together with wooden sticks, a sling for a broken arm, a head band or a bag to carry things. Similar to this Superman's cape is used to carry or catch in the case where he cannot be in two places at once. For instance in one comic I saw him take his cape and stretch it between two buildings to catch people falling from above while he went off to fight the bad guy. Apparently his cape has the same bullet-proof like qualities that he has.
The cape was originally made from his blanket that he was wrapped in when he arrived in his space ship.
In early continuity, Superman's cape was just like him... invunerable. So when his own powers were negated by kryptonite, the cape saved his life by preventing a bullet from entering his skin. (blue kryptonite negates the cape's abilities). He also used the cape to protect people when he flew them about at high speeds.
In the current continuity, Superman's invunerability is a "force field" and his cape is just normal fabric. With this version, clothings which is tight against his skin is protected by that field. Now, more serious comic authors show Superman's cape being shredded in intense fighting. Others kind of let this detail slip and just draw the cape intact after he is hit with "massively damaging attacks".
As mentioned it was originally indestructible as it came from his home planet in manufacturing. I remember him using it to shield him self from rays and stuff whe I was a child.
Like originally he could not fly just leap tall building I think. I guess then the cape would also have helped him stabilize in the air or even glide.
Edited: krakyn on 25th Jul, 2008 - 6:17pm