A sketch acts as a plan for a 3D model. So in case you make a 3D body out of your sketch(es), save it in a standard format like step or iges, your sketch(es) won't be exported and thus when you open it with another software only the 3D body will show up, the sketch(es) won't be there.
A line body acts as a 1D/2D body and thus will be exported along with your 2D/3D surface and/or volumetric bodies.