The name comes from the Latin locus communis, a common place, meaning a general theme under which excerpts could be filed. Unlike a diary, which is chronological and introspective, a commonplace book is thematic and outward-facing: it gathers the best of what a reader encountered so it can be found again and reused in writing, speech, and argument. Figures from Milton to Thomas Jefferson kept them, and Renaissance educators taught commonplacing as a formal method.
The practice encodes durable learning principles. Copying a passage by hand forces selection, the judgment of what deserves keeping, and filing it under a theme forces categorization, an act of elaboration that links the new excerpt to an existing structure of ideas. Rereading a themed collection then juxtaposes authors who never met, which is where original connections tend to appear.
Digital highlighting recreates commonplacing at web scale: highlights are the excerpts, tags are the commonplace headings, and search replaces the index. What the tradition adds to the tools is the habit of returning, since a commonplace book was meant to be read as much as written.