Taking notes while reading—or consuming any sort of educational/informative media—forms a feedback loop that enables you to more easily check comprehension and progress. Putting insights from texts into your own words essentially acts as a comprehension check: if you struggle to re-write a point in your own words, it’s likely you haven’t quite understood it.
In this case, a self-reinforcing cycle is also built. The primary skill involved here is expressing understanding in your own words. By shortening the feedback loop involved in practicing that skill, you’ll also get better (and faster) at crafting good notes, thereby shortening the feedback loop even further and accelerating skill growth commensurately.
References
Ahrens, Sönke. How to Take Smart Notes.