# Writing allows us to deliberately forget
We have Working memory limits, therefore making sure that we can populate our working memory with what's relevant in the current circumstances is necessary for us to be effective. Unfortunately we don't really have a full control over what we have in our working memory, other irrelevant thoughts might linger (like open tasks) and will reduce our ability to focus. (See also: Zeigarnik Effect)
We need to able to deliberately forget about the irrelevant thoughts. Forgetting is a healthy process of inhibiting irrelevant memory. Writing makes us have an external system, and it will help us forget about those lingering irrelevant thoughts.
Related: Storing information externally is not a new idea
# References
Ahrens, How to Take Smart Notes (p. 70)
But thanks to Zeigarnik's follow-up research, we also know that we don't actually have to finish tasks to convince our brains to stop thinking about them. All we have to do is to write them down in a way that convinces us that it will be taken care of. That's right: The brain doesn't distinguish between an actual finished task and one that is postponed by taking a note. By writing something down, we literally get it out of our heads. This is why David Allen's "Getting things done" system works: The secret to have a "mind like water" is to get all the little stuff out of our short-term memory. And we can't take care of everything once and for all right now, the only way to do that is to have a reliable external system in place where we can keep all our nagging thoughts about the many things that need to be done and trust that they will not be lost.
Ahrens, How to Take Smart Notes (p. 99)
Transferring ideas into the external memory also allows us to forget them. And even though it sounds paradoxical, forgetting actually facilitates long-term learning.