Once the hem is sewn, when you fold the pleats the edge of the hem will want to move a little - the visible folds will push the edge out of them, and the folds inside the kilt will require more of the edge as it has to go around the fabric to which is is stitched.

The catch/ herringbone st will allow that to happen.

A standard machine stitched hem tends to look a bit bulkier than a hand sewn one as the machine stitching us rigid and doesn't allow the edge of the hem to move.

Anne the Pleater :ootd: