According to Richard Enos in Greek Rhetoric Before Aristotle (3-7), there are three ends of rhetoric.
Heuristic discourse is the idea of rhetoric as epistemic, that is rhetoric has a generative capacity and discourse aids in discovering and creating knowledge. The generative capacity of language leads to understanding and the ability to make (hopefully good) decisions.
Eristic discourse is the idea of rhetoric as persuasion (quite a popular approach throughout classical times and often modern as well) and discourse advocates for a particular point of view, attempting to advance a position.
Protreptic discourse is rhetoric as a pedagogical tool that is both didactic and gives guidance, associated with rational inquiry and where the focus is direction for instruction. Lectures in the classroom and instructions in the biology lab are protreptic discourse.