This was an answer I posted on Quora on 5/28/2018 to aforementioned question in the title.
Before answering that question, you should first consider what is your purpose for reading the particular item you are reading. Of course if you are reading purely for fun, you likely won’t have any purpose in taking notes while you read (unless you are struggling to remember the character of a story, for example).
If, however, you are reading to learn something, taking notes can help facilitate the learning process in a number of ways. But again, the more clear your purpose is, the more useful note taking can be. Clearly understanding your purpose will help guide you to the answer of whether or not you should take notes while you read as well as what you should be taking notes on.
To more precisely answer your question for why should you take notes while you read, the best reason is that:
Taking (good) notes forces you to recreate (and/or reconsider) the ideas of the author in your own way, challenging you to experience the thought process of the author from your own perspective.
This is essentially how we truly learn…by linking what we already know to that which we don’t yet know…and making a new connection that can, in turn, connect to something else, continuously expanding your mental latticework.
In other words, the note taking process helps give you the mental space for the learning process to happen. Can you learn without taking notes? Yes, but taking notes can make your efforts reading much more impactful to your life.