Writing down as a mean to understanding what we read
The mere act of reading does not imply understanding what we read. On the one hand, the study approach of reading several times the same text in order to memorize it is not useful, because we will forget what we read in the long term. This is called cramming, and is useful right before an exam, but not as a lifetime approach to learning[@ahrens2017How to take smart notes: one simple technique to boost writing, learning and thinking: for students, academics and nonfiction book writers, chapter 10].
Reading many times also creates the exposure effect, in which we believe that we understand something by the mere fact of seeing it several times. I do think that this is stretching the definition, but it serves the purpose of the argument (see also: Multitasking does not increase productivity).
Another approach is to write down notes about what we read. In this way we must translate from one context (the book) to another context (our notes) going through our own thoughts. This act is what allows us to put new knowledge in the context of what we already know. Of course, this all fits nicely within The Luhmann method for storing notes. In fact, Luhmann claims that he wrote notes with an eye on the slip-box, thinking how to make them fit.
Kahnemann: "Brains are machines that jump to conclusions"
Feynman: "They [the notes] aren't a record of my thinking process. They are my thinking process"
These are the other notes that link to this one.