To my knowledge, there's a general idea that either you leave them partly empty forever or you fill them up, regardless if you feel like it or not. In my opinion, it's not good. This is supposed to be fun and exciting (most of the time anyway) and applying that kind of force will ruin your experience.
Now, we are all different, so this method or idea will not apply to everyone, but at least feel free to be inspired.
Several times throughout especially the past year, I didn't feel like starting a new journal or sketchbook. I just needed a break and get away from the one I was currently working in. Here the partly filled notebooks come in handy as I can just take a few pages vacation and return to the one I was supposed to fill. This applies especially to the ones where I accidentally got myself involved in a sort of 'design and layout theme' that I'd like to keep through the entire notebook.
Another reason is that I address several topics where I suspect either my opinion or circumstances will change, so a few pages in the back for later updates in the same notebook is something I really like the idea of. Otherwise the updates to a specific topic would spread to several notebooks.
To some organization and order is a high priority and this would give them a month worth of stress. But to a chaotic person like me, this is the preferred way to work, and most likely also what I'd like to see when I look back at them in 10 years.
Here's a group photo of all the notebooks that are either filled or are WIP's. Most do however only have a maximum of 10 sheets left for future comments to a topic addressed in the notebook.
