Short version: Allows you to make armor you have look like other armor you have and gives storage space for old armor.
It makes me laugh that a content patch does not evoke the slightest bit of interest. But this, something so utterly useless, does. I'm almost, ever so slightly tempted, to give WoW another try when this goes live. Collecting pretty dresses was one of my favorite activities. Er. I mean, collecting awesome and impressive suits of powerful armor that did not include and dresses or dress-like elements such as kilts. Never.
Every now and then I'd think "maybe I should play WoW again", but then I'd realize that my nostalgia was for places like BRD and BC heroics, not WoW as I left it. And then I'd get sad.