What do you use your sketchbook for?

Personally, I have two main sketchbooks one is 9x12 and 14x17. As an artist you need a sketchbook because it keeps you organize. And yes, you can draw on regular white copy paper, but it's hard to keep them all together in order in folder. With sketchbook, I always put the date on the first page drawn, then the last page. And I wrote "From Month/Year to Month/Year on the cover. The reason I did that because I will know the exact period of time I spend on one sketchbook. Moreover, if I need to compare two sketchbooks, I can to see if there is any improvement in my skill level.

There are 7 main reasons I use my sketchbook mostly for.

1) To get some quick concept idea down like thumbnails and comps.
2) To practice drawing, rendering, and shading skill and see my progression.
3) I use it in life drawing session weekly
4) Sometimes I go outside to do some landscape or environmental study
5) To work out on some preliminary composition sketches and new character, props, environmental design.
6) Random sketch down before bed, I often get cool weird idea that way.
7) To quick storyboard sequential instead of using script.

Here are some samples from my old sketchbook (I've not scanned anything new recently so.)