I like to write commentary in the margins, so the dots help me know which books are "devalued" and which are fine to donate or loan out to others.
Multiple dots are an indication I return to a book often. Each time I re-read I take notes in a different ink color and try to record the date in that ink color in the front matter.
Oh man, tangent into one of my favorite library book experiences. I checked out a sci-fi book at the library. It was good I was enjoying it. Then a few chapters in, I found a previous library patron had written nit-picky notes in the margin, poking holes in the author's fictional science tech explanations. And these weren't little one-word exclamations, they were whole sentences written in perfectly legible, almost impossibly-tiny pencil handwriting. Some of them even had little drawn diagrams! It went through the whole book, every hundred pages or so some little margin notes about how such-and-such sci-fi babble didn't reflect how space-time actually works or whatever. It was a hoot, a little bonus on top of the book itself.
This was a special treat because the book itself already uses copious footnotes and cross-references from fictional characters to create a maze. And now a real person added to the effect by trying to make sense of it themselves.