Think of a sketchbook as an extension of your inspiration files on your computer. A sketchbook can be an invaluable tool for designers and eager homeowners.
My sketchbook is kept next to my bed and I often scribble fabric ideas on its pages.
Some drawings turn into my Spoonflower fabrics and some do not. I also use it to work out furniture placement and "think" on paper.
For every house I have owned, I have kept a small book with paint chips, fabric and photos of each room and my ideas for them.
Even if you cannot draw, it is worth a try to consolidate ideas and experience the tactile pleasure of paper and pencil that a smart phone can't copy.
I recently peeked into my a sketchbook that had belonged to my grandmother. She took it along on a trip to Egypt in the late '70s and sketched people, faces, architecture and noted colors that she found interesting.
It is a good way for the non-artist (or artist for that matter) to practice drawing and find out what shapes appeal to you.
Yes, I realize that there are a multitude of digital ways to accomplish these things but it is not the same as purposeful doodling. Didn't you ever just find a pen in your hand and mindlessly scribble only to find that you kind of liked what you had made?
I often sketch a motif from a fabric or porcelain that I like and worry about how I'll use it later.
Saving this visual journal can be a wonderful springboard for ideas down the road.