Sketching on real paper is important. There is nothing wrong with sketching digitally. I just think it is important to sketch with real paper sometimes, maybe most times. Something I do like about sketching digitally is the ability to reduce the opacity of a sketch and through another layer over the top and sketch it again with changes, fixes, new details. When I work with real paper I frequently work on tracing paper so I can easily add new “layers” and do the same. Sometimes when I work in layers and work quickly you never know where the sketch will take you.