Lecture is the preferred method for the economical transmission of information. Discussion favors student feedback, particularly where the issue is one of unsettled opinion, rather than accepted fact. Philosophical literature contains a striking rhetorical form reducible to neither of these procedures, although it bears some similarity to the discussion. I refer to dialogue, particularly as found in Plato's Dialogues. Some of these apparently were based upon real oral conversations in which Socrates was a paramount participant.
Dialogue is found again and again in the history of philosophy.
The dialogues of Berkeley and Hume are famous modern instances of the form. It is arguable that all good philosophical writing is dialogical in form, at least implicitly. First of all, and most obviously, dialogue is conversation, either written or oral, between two or more participants on some issue of genuine importance to the speakers. We can learn much in solitude, but we cannot participate in a dialogue in solitude. Where the issue involved is usually complex and demanding, such as in philosophy, it seems to be a necessary rhetorical device.