That way the edge of the quilt has only one layer of fabric. Any quilt judge will tell you that's a no-no.
This silly quilt police rule needs to be changed. I have quilts that are years old, been washed and dried many many times with single fold binding and the binding looks fine. The single fold binding is not stuffed full either. The quilts that have double fold bias stuffed binding are fraying and showing wear on the binding. I seldom use double fold bias binding anymore. Too fussy and not necessary.