As an example,

You seem to be missing the examples at the moment, but I'll give one... it's damn hard to learn relativity by reading Einstein's original papers. Your average undergraduate textbook gives a much better explanation of special relativity.

On the other hand, when it comes to studying history, sometimes classics are still the best sources. For example, when it comes to the Peloponnesian War, everything written by anyone other than Thucydides is merely footnotes.