As with yesterdays example the sketch is, well "sketchy" in the manner that it's not complete if you compare it to the full orchestration. Only the main structure of the piece gets sketched and it's during the orchestration process that I add most elements and all the other little details.
Another good use of sketching is when transcribing a piece of music, start with a sketch and only notate the most important parts at first. Once you have the melody and harmony down, you'll have the form of the piece and good reference points for moving to the full score later.
In this sense transcribing is very close to composing. The only difference is that you listen to someone else's ideas on a recording instead of your own inside your head. The better you get at one the better you'll be at the other, a worthy thing to spend your time on in other words:)