Discussion boards are used to aid in the extension of classroom content. It provides every student the opportunity to voice their opinion on various topics in which them must apply higher order thinking skills. When discussion questions are posed correctly students are answering questions in which they must apply knowledge learned from inside and outside of the classroom. The openness of topic discussions will offer students multiple perspectives to consider while also forming their own perspective.
I would use discussion boards towards the end of a unit. I would love to see how students can make the connection between numerous concepts throughout a unit. The timing of the discussion board will allow students to review what they have learned and if needed they can research their areas of weakness. The discussion board would not be optional; every student must participate within a certain time frame.