There's a huge difference between sketching and design drawing, and the biggest difference between the two is the need for precision.
Design drawings are created so something can be physically built from the illustrations, while sketches are supposed to be kept loose and quick, a reaction rather than a precise rendering.
Designer Gianluca Gimini thought it might be fun to turn a scratchy sketch of a bicycle into a realistic design, and after five years of gestation he was ready to bring this concept to life.
Gianluca asked hundreds of people to sketch a bicycle from memory on a sheet of paper, then he turned their ridiculous designs into 3D renderings of real (but hardly ever functional) bicycles for his series "Velocipedia".
Even though many of them wouldn't function for long before falling apart, it would be interesting to ride them all and see how the little errors in the sketch affect how each bike feels to ride.