That edge has to bind off so that it complements the cast on edge that was your initial question. Look at the diagram and you'll see that those two edges (the bound off and cast on) form the upper ends or edges of the cardigan fronts. There's no edging on them, just the bind off and cast on. The picked up sts for the fronts will then be knit down from the yoke and that will complete the front opening on the cardi.
Believe me, we all have a lot to learn! Good luck with it and it you have more questions, come back.