Problem: Grant-Davie seems to want us to use the idea of rhetorical situation mostly in an "analytical" way, to understand why existing discourses have taken the shape they have. In other words, he seems to be talking to us as readers. In what ways is the idea also useful for writers? That is, how is it useful to understand the rhetorical situation you're "writing into"?
Add more explanation to the response below:
He is proposing stasis theory be used as an analytic tool and organizing principle in the sequence of questions that explore the exigence of a situation. The states provide a natural sequence of steps for interrogating a subject. This sequence proceeds from questions of fact and definition through questions of cause and effect and questions of value to questions of policy or procedure.