It's presenting the opposite view, even if you don't agree.
Generally this comes up in a discussion that people have strong opinions about, and it is designed to test the other person's logic.
Person 1: God exists.
Person 2: No god doesn't exist
Person 1: Yes god exists look at all the beauty in the world
Person 2: ok God exists, and there is beauty, but why is there sickness, war, and poverty
It is meant to both challenge the person, and keep the discussion going