When you don't agree with someone in a discussion or debate, this expression can be useful. You use it to continue the discussion, even though you don't agree about one point.
For example, you can use it if you and a friend disagree about whether to fly or take a train to travel to another city. You think the train is faster, but your friend thinks it's faster to fly. You can't agree about this, so you don't want to argue about it. But you have other points to make too, so you say:
For the sake of discussion, let's say that flying is faster. The train is still about half the cost.
In this example, you don't really think that flying is faster. You think it's slower, but you want to move on to another reason why the train is better.