Here are a few pages out of some older sketchbooks. I use sketchbooks for character and composition development. What does a bunny look like? How does it look from every angle? These drawings are the character development. You must know how a character is constructed before you can put it into dynamic compositions. This is what I use sketchbooks for.
I think it is very important to my artistic career to have sketchbooks. A tip I heard a while ago is very simple. You should draw an image until you no longer know what to do next. Stop drawing when you can’t figure out how to make it work. Turn the page and start again. You should be willing to do this very frequently. It does you no good to make messes on top of messes. Just move on. You will avoid the tension and frustration that comes with thinking you are failing. When you have 20 sketches in an hour, you can look back and say “Dang! I figured some things out!”