A full calendar can tell you where you planned to be without telling you how much time you spent on the work that mattered. You might finish a busy afternoon unsure whether you moved a project forward or kept returning to messages. A small focus log gives you something concrete to review.
To track focused work time, choose a few categories, start a timer when you begin a specific task, and stop it when you leave that task. Then review the recorded time alongside what you produced. You can do this with a notebook and a timer, a spreadsheet, or a dedicated app. The useful part is a consistent rule for what counts.