@curtis I dont think it has a singular purpose, depends on the situation.

One of the most common purposes is to explore the grey area.. when you have a stance or idea but want to challenge it to see how your stance may change as a result (or to help do the same for the reader).

That said if someones ideas are obviously idiotic and have little hope of providing anything useful to a topic then no one should feel obligated to waste time in engaging.. We all have limited time in our lives, we have to pick the activities we feel will be most rewarding. that means that we have to ignore many attempts at a debate, simply because there are other more productive debates we could invest that time in (or other activities in general).