When you crochet a project in one piece such as a scarf, no seaming is required. But, some projects are made of several pieces that need to be joined together to form the finished piece.
The simplest method to join two pieces is to sew them. This produces a non-bulky seam, suitable for garments.
Working a row of slip stitches or single crochet produces a sturdy seam suitable for joining afghan squares. Worked on the right side, these two methods produce a decorative ridge that can be a nice addition.