John Milton kept one, and I keep one. A commonplace book is a great method for taking notes when one comes across an interesting song lyric, a poem, a quote, a conversation overheard, something witnessed, a random idea that hits with little or no provocation or warning.
These bits can be put down in text form, sketches, or both. Whatever works best for the notetaker at that stitch in time. Referring to a commonplace book and its contents weeks, months, or even years after the ink was set on paper, can be inspirational and instructive. "Oh, that's what spurred me on."
I'm good at making mental notes, but more often than not, if it's "not written down" when the bolt strikes, it eventually, at times too quickly, gets lost with the storm as it moves on.
If you read a lot, a commonplace book is a good companion; for writers, this book is essential, or at least it should be.
Above is a sample from my commonplace book. As indicated at the top of the page, I sketched this bit on September 19th, 2015 while reading Alfred Price's 1976 book The Bomber in World War II. Rereading the pages now reminds me how air power has its limitations. I'm interested in military history, so the quote is in synch with my interests... which is generally a hallmark of one keeping such a book.
More pages to come....