The goal of Mermaid on the short box is to build strength of the Obliques and the QL (hence the "sexy V") as well as increase the range of motion of the lateral spine. Mermaid on the Box is a big exercise bang for its buck! You also work Abductors of the hip (glute medius & minimus), Latissimus Dorsi (back), Anterior and Middle Deltoids (arms), Transverse Abdominals, pelvic floor, and shoulder girdle.
Yes this lovely exercise can be the Bain of our existence, but with a little persistence and refinement to lengthening (think up and over, vs. only side bend!) while stabilizing, we learn to appreciate its complexity.