This is called thread tracing. It is used in creating couture clothing. The thread tracing is done to show where the stitching line should be. This allows the sewer to cut out the pattern pieces with various seam widths. For example, suppose you have concerns about the hip width in a design. With this method, you can cut out the pattern and add as much as 2-3 inches to the seam width. Then if by stitching along the tracing your skirt is too tight in the hip, you can simply tear out the seam and trace a new stitching line that gives more room in the hip.
Most home sewers do not find the time to thread trace their pattern pieces and the standard seam width for home sewers is 5/8". These sewers simply cut out the pattern and use the seam width markings on their sewing machines to determine where the stitching line should be. So, here is the rub: Suppose you use this method and your skirt is much too tight. You can rip out the seam, but you can only add about 3/4" to each seam. (Need at least 1/4" on each side of the seam for seam purposes; this leaves you with 3/8" on each side of the seam to add to width or 2 X 3/8" = 6/8" = 3/4") Obviously, this is a problem if the skirt is 2-3 inches too tight, and it is a BIG problem if you do not have enough fabric to cut out two more skirt pieces.