These markers will give you 3 areas: right shoulder, center neck and left shoulder. It’ll help when you are ready to seam the right and left shoulders to the back. The center neck will be part of the stitches picked up for the neckband.
If you don’t want to work the mattress stitch to join the shoulders, you can use a more conventional sewing seam. In that cast you would pin the shoulders to the back with the public or outsides facing. Stitch along the edges like this (5:30 minutes for the stitch):
You would be seaming across the bound off edges, not the side seam but the method of forming the seam is the same.