The article points out that with most to-do lists, people try to scratch off the most items as possible, often resulting in people taking care of the quickest, easiest items, leaving the bigger items for later. By tracking what you’ve accomplished with the “done list,” the goal is to figure out your patterns and improve your productivity.
It’s an interesting idea, but I don’t know if it’s totally necessary. So long as you prioritize your list at the beginning of the day and review your list at the end of the day, your patterns should be obvious–all without the need of a second list.
But the important thing is to develop a system that works for you, and then stick to it. So that’s why I present this. Another arrow in your productivity quiver. Good luck!