If presented in an honest way, it can be a form of open-mindedness.

I’m not sure how to say it more gracefully, but it could be something like:

I’m willing to grant that assumption even if I don’t believe it, in the context of this conversation, in order to explore the implications of that assumption.

Sort of an application of the scientific method, in combination with an acknowledgement of how the unconscious works in regard to mental resources. People often won’t really think about something seriously unless they believe its real. But in order to test a scientific theory one must use it to generate predictions.

So sort of pretending something is true, pretending you believe it even if you don’t believe it, could be a way of activating all the resources of your mind to explore where it goes with the implications.

I guess one way to put this from the other person’s perspective is to say “Humor me for a second. Let’s say that …”