A commonplace book is a scrapbook where the author can record various important pieces of information that they want to remember. They were popular throughout the ages and were especially popular in early America, as paper and books were much scarcer than they are today.
Many commonplace books held facts, recipes, thoughts, weight conversions, important dates,letters and newspaper clippings. The act of keeping a commonplace book was called "commonplacing" and the technique was taught at Harvard so the students there would remember more. In a time before quick and easy information access, these commonplace books could be a lifesaver when a certain address, medicinal recipe or quote was needed for oneself or to share with a friend.
These weren't so much journals or scrapbooks but a mixture of both. Unlike journals, they contained some information copied from other sources. Their main value was to aid the memory of the author but are particularly interesting to historians because it gives a glimpse into what a particular author found interesting or thought was important.
Commonplace books exist for many historical figures such as Henry David Thoreau, Thomas Jefferson, Even as books became more prevalent and cheaper, many people still kept commonplace books because of their advantages.
Commonplace books facilitated memory and the sharing of ideas. In modern times, the use of a commonplace book has been compared to sites like Pinterest, where users can "clip" different websites, pin them on a virtual pin board and share them with their friends.
Even in modern times, a commonplace book is an interesting idea because you can see how your interests change over the years. It's harder to keep track of the changes that occur in ones interests using the internet and it is also not something you can keep in the family. Commonplace books are one of those things that have value to the people close to you but would be seen as pointless to anyone who didn't know the author. I think it is still important to create physical records especially as we create more and more digital records. Digital records are great but there is something special about being able to to hold a family keepsake.