I have recently dedicated some time to going over a stack of journals that I've maintained over the past few years. The purpose for the review is to check on notes about a particular project; thoughts jotted down at random and yet with a common theme.
They are a treasure chest and an overall reminder of the importance of having a journal in hand so thoughts can be quickly preserved.
Because we all know what happens when we say, "I'll write that down later."
[BTW: Although some of the journals are inexpensive ones, my favorites are the Moleskines and the Dingbats. You can't go wrong with either brand. A Dingbats Wildlife journal is shown above.]