Cecilie Bahnsen white dress size 2 BNWT

Cecilie Bahnsen white dress size 2 BNWT Sleeveless nylon-blend short dress in white. Square neck collar. Gathering at chest. Welt pockets at waist. Criss-crossing shoulder straps and detachable self-tie belt at back. Zip closure at side-seam. Full silk crepe lining. Tonal hardware. Supplier color: White Body: 82% polyester, 18% polyamide. Lining: 100% silk. there is a part of of seam that is not stitched, see last picture. I can't figure out the purpose of it pit to pit 15" lenght 38"