I scribble quotes that mean something to me, all through my sketch books.
Sometimes I even put them on my blog! This sketch book page ended up drawn on, painted, collaged, painted again and then a quote scratched on top in sharpie. When I put things in my sketch book, they are never for show. My sketchbook is private and strictly for myself to capture ideas for a later date.
I feel it's also written in a personal language that's all my own, no one else would ever be able to decipher it anyway. Every mark I make is connected to larger, well developed ideas and concepts that reside in my mind. I guess the sketchbook is more like an index page. The real content is within me.