What the notebook knows
The useful thing about a notebook is not that it remembers. It is that it disagrees. Memory is a diplomat; it smooths the record until the past appears to have led sensibly to the present. Paper has no such manners.
I went back this week to entries from four years ago, expecting continuity, and found a person confidently certain of two things I now think are false. There was no gradual revision in between. The change happened somewhere off the page, and I would have sworn under oath that I had always believed the later version.
That is the whole case for keeping a record: not nostalgia, and not productivity, but the ability to be contradicted by primary evidence. It is the cheapest available protection against becoming a person who has never been wrong.