Hi! thanks for the question. This is a standard sewing practice and it’s the same thing done on collars as well. The reason for it is because when doing the edge stitch, you should be sewing it looking at the right side of the garment as the top of the stitches are usually slightly better looking than the stitches on the back. No matter how perfect you stitch, sometimes just through sheer accident, the edge stitch doesn’t always catch the top of the inside band. So the slip stitch is doing two things. It holds the band together without having to worry about pins on the inside and if the edge stitch doesn’t catch it in a few spots, it’s still completely sewn in place. The edge stitch is really meant to be a finishing stitch. Hope that helps! 🙂