For me quilting is a technique of it's own. You can combine it with piecing or appliqué in many different ways, it can highlight the design of your quilt top and enhance it - but quilting can be used alone as well in wholecloth quilting. Piecing and appliqué uses color to create contrast, quilting produces structure and variegates with light and shadow - the color of thread complements both.
You can be very creative with your quilting - it is much more than just fix the layers of a quilt together.