While RTFM is technically a correct answer to any Emacs question, the reason for purchasing a secondary resource is that the author has presumably curated the documentation for a particular audience.
I agree that Emacs documentation is very good, and once I recently got in the habit of C-h'ing for answers rather than Google'ing I became more productive and can work more productively with the WiFi on my laptop off [not having the temptation of HN among the productivity enhancements]. But this doesn't change the fact that better explanations are possible and that a teacher is often better than a specification. Experts can contextualize a problem, and that has added value.
I agree that Emacs documentation is very good, and once I recently got in the habit of C-h'ing for answers rather than Google'ing I became more productive and can work more productively with the WiFi on my laptop off [not having the temptation of HN among the productivity enhancements]. But this doesn't change the fact that better explanations are possible and that a teacher is often better than a specification. Experts can contextualize a problem, and that has added value.