When you start a top-down shawl you begin at the very center and work increases out from that point. This means that if you start straight from a cast-on, you will have a 'gap' at that spot where you begin working the increases and edging out to each side.
A garter stitch 'tab' is used to effectively cover over this spot and give you a little triangle of knitting to work around. You'll cast on the number of stitches you're working for your edging (typically 3 or so), work several rows and then pick up the bottom stitches and pick up the cast-on stitches.
You can even use a Provisional Cast-On if you want to avoid any seam at the start. In a pattern, it would look like this:
CO 3 sts. Knit 6 rows. Do not turn on final row, rotate corner 90 degrees, pick up 3 sts along purl bumps, rotate corner 90 degrees & pick up 3 cast on sts.
You can see a video of it in action here:
