Read Sketching: the Visual Thinking Power Tool and The Miseducation of the Doodle. I should have read these before working on my self-portrait because they explain the importance of sketching/doodling/memo/…/whatever. But it's ok I can start from now.
The idea is pretty simple: sketching/doodling help to generate/explore ideas and sharing them with others. I agree with the idea from my experience.
I'm wondering which medium to use for sketching. I mainly use iPad for note taking but I do not always carry it, and I to not carry a note book too. But you never know before you try, so I'll try both, iPad and note. Eventually I'll find out which medium I like.