The batting, even if just a layer of flannel helps give something for the quilting stitches to bite into. It also helps hide the color of the backing from showing through to the top through light fabrics.
I think quilts without batting are often called coverlets or summer-weight. I have rescued some from thrift stores and garage sales. Often they are "knife edge" with a blanket stitch around the outside, probably "birthed" -- when you sew together the edges right side in, and then turn out so the seams are inside and the right sides of the fabrics are shown.