Think of it kind of like those "bubble skirts" (like this
) - the "poof" in the skirt needs to be held up by something, so there's a non-poofy layer under there it's attached to. This will stop it from being able to slide up and down the arm - otherwise the "puff" part of the sleeve will want to move and collapse a lot because it's not supported by anything - kind of like if you took the support layer out of that skirt, it would all just fall down straight like a normal skirt.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan