Mystery of Two Pieces (Dress with gathered skirt)

I was wondering why some people made 2 pieces dress (with skirt part is ruffled or gathered). Why not 1 piece? It’s annoying to use it on doll. But since, yesterdays, when i made a dress, i started thinking two pieces dress would be better than one pieces. It may look better on 2 piece rather than 1 piece. The waist part on torso wouldn’t be too bulky. It’s hard to explain because the skirt will be ‘slipped’ under the torso waist. Argument may arise, it is the same. Bulky is still seen. Yet, somehow, I still feel, now, 2 pieces is better than 1 piece.