It has many porpeses, first the teatrically and part of the Bat-thing. But check out this article
http://en.wikipedia.org/wiki/Batsuit#Cape

Batman as of the name the cape indicately represents Bruce Wayne to manuver and GLIDE towards anything within range. It's also an addition to the costum itself so fashion check THERE!!!!!!

It is mostly because he needs to look lke a bat to spread fear to criminals of Gotham City. Originally, Bob Kane wanted Batman to have wings but Bill Finger changed it to a cape (thank god)

To help shield him, make him look more intimidating,and he can use it as a weapon. besides who doesnt love wearing capes?

well...
if he didn't have a cape he couldn't jump off buildings (wich is AWSOME!)or look like a bat.
duhhhhhhhh

Goes good with his boots, fierce!!

Batman needs a cape so he can glide through the air... plus he looks more like a bat.

so he can glide

He needs it to glide.

Batman wears a cape because:
1.He can use it to frighten criminals
2.It is a lot better to glide than to fall to the wall.
3.He can use it to cover himself from fire.