Because even if Dragons don't really need clothing, it helps a questing adventurer to look more presentable.

Fun fact, I later made a partial of this: https://www.weasyl.com/submission/691156/blades-to-the-sky

Original:
http://ageaus.deviantart.com/art/Battle-Roar-217265991
http://www.furaffinity.net/view/6078739/