I have always been a huge fan of quick and lose sketching. The ability for an artist to capture the movement and the emotion of a figure within seconds is something to be admired. It hones the artist's skills and develops a quick eye for detail. Not just any detail, but the important detail. That and it's art in it's truest and rawest (is that a word?) form. 
Why else are there so many "The Making Of (insert movie title here)" books? Most of us want to see the inception.  Like a sketch book of one of the masters: L. Da Vinci, M. Angelo, N. Rockwell, T. Avery.