If you are meaning a "facing" I use a facing when I do art quilts, where I want a nice, sharp edge without the intrusion of another fabric around the edge, which a binding would make.
It's just another technique that can add to your quilt, depending on the look you are going for.
Otherwise, the only time I use interfacing is when I am using specialty stitches on my tops, so that the stitches lie flat.
Watson