Part of our job is to help them learn to do that, to help them learn to decide what they need to do to be effective when writing on a given topic for a given audience. Those decisions should always guide the writer's sense of form. Of course, there is plenty of other guidance available. One value of reading as a writer is to see how other authors writing to similar audiences, on similar topics, have organized their writing. That is one reason we use a reader rather than a rhetoric in English 101.
Readers allow writers to witness the effects of rhetorical decisions other writers have made.
Rhetorics often simply provide more forms and formulas removed from actual writing situations. A good deal of writing has the form of generalization and support. The writer has a progression of points he or she wants to make and includes support for each assertion, sometimes with one idea and its support in each paragraph. If this works well, the reader feels like he or she is being led in some meaningful progression, from one idea to the next. The point is that the writer's organizational design should be clear and natural for the readers.