So let’s say, let’s say, for the sake of argument, that all of the water levels around the world rise by- by, let’s say, five feet over the next hundred years. Say, ten feet, by the next hundred years.
(Video, climate change)
For the sake of argument is a fixed expression. It means something like "let's accept this hypothesis as a basis for discussion". We don't ask to believe in the likelihood of an event, we only ask to consider it as a basis for discussion.