I love this idea.
Sometimes I sketch to remember things -- birds right now, but also memories of trips like this one:
Other times I sketch to explain things to others, like this diagram of the four eras of computing:
Ever since reading the Zen of Seeing in prep for this project, I've started to sketch as an observational activity. Here's the first drawing I did that I looked down in amazement at what I'd seen. It is of a friend who was sitting beside me, also drawing.
And more recently, I've begun to play with watercolor. My current favorite is a drawing I made while on Primrose Hill in London.
I think mostly, I sketch to express what's in my head (either what it's processing or wanting to communicate) in ways that words can't express. I also encourage others to sketch because I see it as a pretty innocuous way to take a creative risk and I think creative risk-taking is something generally good for humans to do.