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.