This is actually the question the product answers. A lot of graphic designers like to start designing on paper, then move to Illustrator or PS for the final work. The classic way to do this is to scan in your sketch and make it a layer, and essentially start over with digital art on top of it.

Inkling lets you split your sketch into multiple layers as you're sketching it. Say you're drawing a character for a game, and the character can be carrying things. You can sketch the two together, with overlapping lines, but keep the character and the carried thing on separate layers. All without having to find the boundaries and snip carefully to separate them in PS.

Sheer convenience, too: if you draw with this pen, the drawing is already scanned, rather than having to dig out the scanner (which, in our household, lives in a pile of other gadgets under a desk) and fiddle with TWAIN settings to get stuff into Photoshop. Plus you can work on the move if you have your laptop and a sketchpad with you - of course, you can do this with a regular graphics tablet, but from my (hobby) artist partner's comments, it's much more fun drawing on paper than on a tiny graphics tablet.

Layers, automatic vectors, pressure sensitive tips... not really stuff that you can trivially replicate with just paper/pen.