Externalizing thoughts
Thinking in writing is about externalizing our thoughts to any other medium. As we’re limited in ~ seven items to remember in our short term memory (STM - Miller, 1956), writing helps unclutter our STM and keep free space available to store more important things to remember.
Externalizing our thoughts also helps us fighting the Zeigarnik effect that state the tasks’ propension to fill our mind until they’re finished or written down.
Externalizing thought could be on paper, on a todo list if it’s a task, or in our second-brain or Zettelkasten slipbox.