Having a 5/8" seam for clothing allows for using a variety of seam finishes. I cut a much smaller seam allowance when cutting the final garment, but always the larger when making a muslin, for the obvious reason of I don't know what will work and need room to play. Most of the time I cut a 1/4" seam allowance for my garment clothing. In areas of 'stress', I finish the seams together with a zigzag stitch and the 1/4" allows for that (like a serger would do).