Sketchbooks always made me nervous... I started to keep sketchbooks after graduating art school, and only then realized my visual musing, on my own time, nudged me into recording things I found interesting to draw. Choosing your subject matter, materials and compositional criteria becomes a personal endearment, and once established... your point of view clarifies. It belongs to you. Once you've committed yourself to the fundamental discipline of "drawing what you see" one captures the expansion of abilities to draw "what is not there". This is true, somewhat mystical, always personal, a portal into your own world.
Sketchbooks become an autobiography of one's placement in time, a vessel within to draw something good, or bad, in a hundred different ways. An aerobic exercise for the hands, your eyes, and your brain. Whether on paper or in a pixel-based format, sketchbooks bring forth questions about yourself. Where did that come from? Why did I draw that? Is this any good? Is it art? Sketchbooks still make me a little nervous...