I just bumped on this thread while i was working on a project and i thought it was worth answering to your question.
It's a real coincidence because in the contest i've just joined, the Contest Holder is specifically asking to show the work in progress from sketch to digital. I will not only keep the sketches, but add screenshots of my work in progress too.
IN my practice i do this not just to show that the work it's mine and original, but because i want to show the potential client the process my artwork creation.
The client get a better understanding of the work process and the amount of work i put in my job (sometimes underestimated from clients).
I think this part of the process can help both client / designer in case of changes are needed as the client has a better vision of the whole work so that the changes can be done quicker and more efficiently.
I agree with Vincent post. I myself get a lot of inspiration from old sketches i used for other projects. Sometimes i take 1 or more elements of a particular sketch or i redraw the same sketch again and in the process a new illustration come up. The only downside of this process is that
I keep everything rough and better sketches. I leave my studio has paper flying every where! So once insketches close to my desk on top of a while you haveshelf and easy to tide up because your work environment can become very messyaccess. This is important because it's quick to review them. If i don't use this system it would be too annoying and wasting time to look for all the doodles around the house and i'll end up never looking at my old works. you'll drawn in paper!!!
The only downside of this process is that my studio has paper flying every where! So once in a while you have to tide up because your work environment can become very messy... you'll drawn in paper!!!
This is my 2 cents. I hope it helps.