Everyone should keep a commonplace book. This is especially helpful for people who tend to borrow media from the library or others. By keeping a commonplace, you will still maintain your favourite lines or excerpts of something even when it's no longer in your possession. Beyond that, commonplacing is a way to see yourself through the life of others. It makes it easier to understand yourself through time. I struggle to see the perspective of my younger self, but when I read the lines he chose to point out, I finally understand.