In your case it sounds as if the fusible interfacing is used as an additional backing to keep the fabric under the applique from showing through. I use this when I have lighter appliques over darker backgrounds. I just iron the interfacing to the back of the fabric I am using for applique and then cut out the shape-it becomes a "new" piece of fabric that won't shadow through. You can still use the fusible web if you are wanting to stick the applique onto the background.