Most painters, or even sculptors, will tell you that they spend most of their time 'sketching'; not creating 'great works'. We scratch about with pens and pencils, recording the smallest and most unimportant of things, just in case they should come in handy at a later date.
Above is a good example. A quick-fire sketch that I came across in one of my books, that eventually got worked-up as a painting.
This VERY rough drawing (above) has yet to be used anywhere, but who knows! It may not appear in its entirety, but just the positioning of a window might find its way into some composition.
I hate to think how many sketchbooks I've filled with inconsequential drawings, but I consider every single line to have been worthwhile. Even the half-dozen lines of this sleeping dog might, one day, turn up in the corner of a much larger work.
Most painters protect their sketchbooks with their lives. They ARE their lives, and can never be replaced. But on account of their nature, you'll rarely be invited to look inside; they mean nothing to anyone other than the hand that made them.