I'm not going to speak for every artist, but for me, sketchbooks are a necessity in order to work out my ideas. They also serve the purpose of acting like a "journal", but with images instead of words. Sometimes I will just sit with a blank page and "doodle", other times I will have a few reference images that I like that will be the basis of my sketches.
Sketching is important because not only does it help hone your drawing skills, but it also allows you to tap into the creative side of your brain. Since the drawings are only for you, there is no risk of failure, and you can do the most experimentation and figure out what works and doesn't work. This allows for innovation in your regular work. It helps you to push your boundaries, explore, and become an overall better artist technically and creatively. It's kind of lying playing basketball and shooting free throws in your spare time. The more you can do it, the better it will help your real game.
For me, I try to work on one sketchbook at a time, although I have in the past kept different sized sketchbooks for larger drawings and smaller studies. I also use the program Procreate on my ipad as a sort of sketchbook where I can do drawing exercises when it's more convenient. For me, I also use my sketchbook for preliminary studies of larger works. I am able to problem solve the layout, colors, and other issues related to the final image. Doing this ensures that I don't run into any unforeseen issues when working on my paintings. I hate getting halfway through a painting, and then realizing that the colors or overall design don't work. Over time, you want your work to be as strong as it can be, so things like composition and color become more and more important.
Finally, I also use my sketchbook to write down new painting ideas, plans for the future, and general to do lists that are art related. Having things handwritten in my daily sketchbook allow me to see my notes and not just have them written in a folder that gets lost on my phone or computer.
I would recommend a daily sketchbook for artists or hobbyists of any skill level. They are great at helping you get out your ideas and problem solve your work before you even begin. They allow you to figure out ways of being more creative, and to think critically about your work. You can play with designs, layouts, colors, even different media. I like a spiral bound sketchbook, because I will sometimes need to rip out pages, and being able to fold the book in half to create a flat surface is important to me. Other people prefer a bound hardcover sketchbook. You don't need anything fancy, but just remember that if you plan to experiment with different media, you will need to put a scrap sheet underneath your page, otherwise the material could bleed onto other pages.
Thanks for checking out my website and my blog. I plan to write more entries about my process, types of art that I create, and my general thoughts on art and creativity. If there's a topic that you'd like me to write about, you can contact me at firstname.lastname@example.org.