Hand sketching is still the best way of getting ideas out of the designer's head and onto paper quickly and efficiently. 3D modeling requires manipulating software to get the result that the designer wants. This process is often slow and cumbersome. Manipulating your hand to draw the line you want it to draw is much faster and easier.
To build a 3D model of one concept can take days. To do a hand sketch of one concept can be done in an hour or two. This means many ideas can be explored far more efficiently and less time is invested into each idea. Often when concepts are presented for consideration they are dismissed instantly. If little time has been invested into that concept, then little has been lost.
Even in high end design programs, such as Alias that many automotive designers use, the base drawing is done by hand on a digital tablet. Even though the drawing is done directly onto the computer through the tablet, it is still the designer's hand making the moves. As this is most direct way to bring an idea from inside their head to life.
Remember, computers are just another tool like pencils and markers. If you want to be a car designer, practice your freehand car drawing skills and you will not only produce more concepts but better designs as well.