People understand teaching through more than individual words; familiar modes of discourse also matter. If instruction is delivered through a form wholly alien to its audience, the subject itself may become harder to access.
Speeches, public addresses, correspondence, and poetry had recognizable modes in early Arabic literary culture. The Qur’an addressed ears familiar with that linguistic world without confining its content to the limited structure of a subject-specific manual.
Conclusion: intelligible presentation is part of effective guidance. A scheme that is convenient for academic cataloguing is not necessarily the best form for every act of exhortation or instruction.