When you do so - you ensure that the content of the notes uses the same mental models, analogies and examples as your brain does. Making incorporating the ideas from your notes into your thinking far more seamless as compared to just storing a reference to a third-party source.
The act of translation also allows you to [test and improve]([[write evergreen notes to get an immediate feedback on your understanding of the idea]]) your understanding of the idea as you are reconstructing it with the words and concepts you use to look at the world. It moves you a bit closer in the direction of truly internalizing the new concept
Doing this is in a way an exercise of [[Benjamin Franklin method]] for writing - you take the ideas expressed by someone else - process them and write your own summary of them. You can compare results to your sources and the way other people expressed the idea, as a source of feedback.
Synthesized the insight from the chosen fleeting notes into Evergreen by translating them into your thinking context and elaborating them evergreen notes are the ideas or a thoughts elaborated in their full form