My sketchbook is very important to me. It allows me to write down ideas and sketch concepts. Keeping track of thoughts and reflecting on past projects allows me to grow and push my ideas further. Even if a past project is 'bad' in my eyes, it is not a mistake. Nothing is ever a mistake, it's a learning experience.
My digital sketchbook is a collection of process work, old work, behind the scenes photos, concept illustrations, and experimental pieces.