First saw this joint used in a bishop's shephard crook - to make the top curved part. The scarf joint part of the joint provides a lot of face / edge grain glue area. The "one end open" mortise and tenon keep the parts aligned and the peg just adds insurance. The two variations of mortise and tenon in conjunction with either a vertical or horizontal peg doesn't require ANY glue to hold together. These are Chinese versions of the joint - reversible and no glue required.