This is a questioning technique that seems so obvious when you understand it that you might find yourself using it every day for the rest of your teaching career.
What is the point of this discussion? Well, there are probably at least two things I want to happen:-
1) I gather the thoughts and ideas of my learners, helping me to assess their understanding and giving me a chance to probe and develop their thinking.
2) I want to make sure they are aware of and understand some specific, important points.
A funnel is wide at the top and tapers at the bottom. Using the questioning funnel means starting "at the top of the funnel" with at least one open* question, then narrowing down to draw out specific points with closed* questions later.
Responses to the open question at the start help me to assess understanding and gather ideas (e.g. "What did you notice about the poem / oil refinery / experiment / match etc.). It's very likely that the learners will spot something that I haven't thought of, and didn't intend to discuss. Great! The discussion will be all the richer for it. NB. Notice that the handle is at the top of the funnel, and it is this "open" section of the discussion that helps me get a "handle" on learners' understanding.
The specific point I wanted to draw out might well come out without prompting, but if not I can move down the funnel to more directed, but still open questions (e.g. "What can you tell me about the use of language in the second stanza?") and finally to more closed questions ("What was the intended impact of the use of assonance in line 5?" or "What is the name of the technique the poet uses here?")
Why does this technique make a difference? Well, everyone can have a go at responding to an open question (maximising potential participation in the discussion) whereas only learners who know the answer to a closed question can participate. More importantly, your learners will often respond to open questions in ways you did not anticipate, and frequently these responses will have value over and above your "intended" discussion.
Finally, always be wary of asking false open questions, where you ask what seems like an open question but are only looking for one response. This is effectively playing a game of "Guess what's in the teacher's head" and is a bit of a waste of time.
*For a very brief description of open and closed questions see this Wikipedia entry.