Open tasks occupy short-term memory until they are done. It’s distracting to have so many open tasks because it’s natural to keep thinking about them.
However, additional reseearch by Zeigarnik found that we don’t actually need to finish tasks to no longer think about them. As long as tasks are written down and trust that it will be done later, it’s enough to convince our brains to stop carrying it around in our short-term memory.
From How to Take Smart Notes.