This is zettelkasten’s best practice when reading. Niklas Luhmann never underlined sentences or wrote marginalia. What he did was take fleeting notes about things that caught his attention in the text and then processed these notes into evergreens but thinking about their relevance to his own thinking.
According to Sonke Ahrens, writing about what we read is “the ultimate test of understanding” because it shows how much we can clearly and concisely express an idea that we can use in the future. Aside from this, another added benefit of writing permanent notes is you don’t have to return to the same reference again and again. You just rely on your permanent notes.
To write permanent notes about what you read, see how to convert literature notes to permanent notes.
References
Ahrens, S. (2017). How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking – for Students, Academics and Nonfiction Book Writers. CreateSpace Independent Publishing Platform.