it depends on one's goals.
I leaned a lot of physics without ever having actually read Newton's Principia or Einstein's landmark 1905 paper on special relativity. I certainly never read the original research papers of James Clark Maxwell or Niels Bohr, nor were they required reading in any physics class I took.
On the other hand, I have made it a project in the last three years to learn something about 2,500 years of western and eastern philosophy.
That simply does not leave me the time to read the collected works of Plato, Aristotle, Laozi, Mencius, Boethius, Abelard, Aquinas, Descartes, Locke, Hume, Kant, Hegel, Kierkegaard, Nietzsche, Sarte, Heidegger, etc.
The only realistic way for me to approach this is to learn about these thinkers and their philosophies by reading survey books and learning from scholars who are subject matter experts.