Skeletal animation: why Bone & Skin have to be same level children?

I have a skin (node) with 1 bone. Why is it necessary to attach BOTH
the skin AND the bone to some parent (node) for animation to work? One would think that
since bones are attached to the skin with addBoneInfluence() and setSkeleton(),  attaching
only skin to model (node) should be enough.

Node model = new Node("model");
SkiNnode skin = new SkinNode(mainBone);
model.attachChild(mainBone);      // if omitted, skin shape is visible, but does not animate
model.attachChild(skin);      // if omitted, skin shape is invisible

I think that is to achieve skin and skeleton having the same local coordinates reference system.