The Zeigarnik effect etablish that our brain stay occupied with a task until it is done or written down and better remember of unfinished or ineterrupted tasks better than completed tasks.
This is why todos are so useful: by writing tasks down, we’re fighting the Zeigarnik effect and free our Working memory.
This is one of the effect that make Multitasking counterproductive, as we fill our head with multiple tasks at the same time and try to switch between each. As Focus isn’t what we do but what we don’t do, we better have to write down opened tasks to get rid of the Zeigarnik effect and focus on the tasks that matter.