The importance of different stitch techniques-
Stay Stitch- Wenever you need to clip a curve or cut to a point, it is a good idea to stay stitch first. Stay stitching is a line of stitching that is sewn just to the right of the seam line in the seam allowance, as close as possible to the seam but on on it.
Understitch- Understitiching is mostly used to keep facings and linings from rolling to the outside and showing. It also helps to keep edges flatter. To understitch, clip curves and grade the seam if necessary. Move all of the seam allowances and the lining or facing to one side, Lay the garment to the left and the seam allowances and the lining or facinging to the right. To the right of the seam line and as close to the seam as possible, stitch the seam allowances and the linging or facing together. It helps to use a zipper foot.
Stitch in the Ditch- Basically, it is sewing on top of a seam that already exists. It is very useful for attaching waistbands and sewing some parts of the linings to the garment from the right side.
Source- Twinkle Sews by Wenlan Chia