Sketchbooks are a great way to store all your ideas and develop your work. They do not need to be a piece of artwork but a place to jot down ideas/notes, a way to save pictures/photos, somewhere to record tests, storage for samples and most importantly place to refer to when you need inspiration.
I use a large amount of my own photographs in my sketchbooks as I have always enjoyed the art of photography in its own right. I collect images of details, textures, interesting forms and anything that grabs my attention. As well as storing these digitally I surround myself with these images alongside my tests and collected images by other artists I admire.
You don’t even have to use an actual official sketchbook – you could just use a book, paint over the pages first and then you can start to play to your hearts content over the top with ideas. These can then be developed or just left as a visual stimulus for something else. Or you could just use this as a fun way of creating a record book.
The key thing to remember is that the sketchbook is not the finished piece of art it is a method of helping you to reach a goal and no one is gong to judge you on how you choose to use it. (or even if you don’t use one at all)